Thermosetting Polymers from Renewable Resources

by

Bernal Sibaja Hernandez

A dissertation submitted to the Graduate Faculty of Auburn University

in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy

Auburn, Alabama August 06, 2016

Keywords: renewable resources, polymers, thermosets, biorefinery, chemical modification, thermo-mechanical properties

Copyright 2016 by Bernal Sibaja Hernandez

Approved by

Maria L. Auad, Chair, Professor of Chemical Engineering Mario Eden, Professor of Chemical Engineering

Sushil Adhikari, Professor of Department of Biosystems Engineering Brian Via, Professor of School of Forestry and Wildlife Sciences

Mahesh Hosur, Materials Science Engineering Department

ii

Abstract

Polymer science was born as an answer to the need to produce and to understand materials such

as plastics, rubber, adhesives, fibers, and paints. It has been recognize as an interdisciplinary

field, combining theoretical and experimental knowledge from areas such as chemistry, physics,

biology, materials and chemical engineering, etc. It has also been an industry of high levels of

innovation, leading the major investments in research and development programs. The

continued use of finite fossil fuel resources has shifted thinking towards the future energy

scenario of the world, and the polymer science has not escaped to the impact of this trend. The

interest in renewable resources is constantly increasing, backed up by new environmental

regulations and economic considerations. Biomass is abundant, and polymeric materials based

on renewable biomass feedstocks represent a viable alternative to fossil resources. The present

work aims to synthesize new, potentially bio-based thermosetting materials from biomass-

derived compounds, use them to prepare bio-based thermosetting resins, and investigate the

structure-property relations of the resulting polymers. In the first Chapter, background

information is provided about the current status of industrial biomass, emphasizing on numbers

as well as on new governmental regulations aiming to impulse the use and study of biomass as

an alternative chemical feedstock. In the next Chapter, the cationic copolymerization of tung

oil, employing limonene and myrcene as comonomers, is presented and discussed.

Characterization revealed that the resulting materials behave as elastomeric materials, with the

glass-rubber transition occurring at room temperature, and modulus values in the order of MPa.

iii

Chapter III investigates the chemical modification of two different functional groups of biomass

derived materials; the carbon-carbon double bonds of triglycerides, and the hydroxyl groups of

phenolic compounds. These chemical modifications were addressed to introduce more reactive

functional groups able to polymerize and to produce materials with a better thermo-mechanical

performance. Testing revealed the deep impact that the chemical modification brings up to the

thermo-mechanical performance of these materials, generating resins with properties similar to

those shown by commercial resins.

Chapter IV considers the preparation of interpenetrating polymer networks or IPNs, a particular

class of hybrid polymer consisting of two (or more) crosslinked polymers. These

multicomponent polymer systems seek to combine the best properties of two or more different

polymer networks in order to achieve a material with better properties compared to their not-

interpenetrated counterparts. Materials with high toughenability were produced and vastly study.

Finally, in certain thermo-mechanically demanding applications, aromatic raw materials are

needed to impart the required properties to the thermoset. The most available source of

phenolics in nature lies in lignocellulosic. Molecular aromatics can be obtained on a large scale

by means of new and arising biorefinery technologies. Chapter V evaluates the viability of

synthesizing a high performance bio-based epoxy polymer using fast pyrolysis bio-oil

(containing lignin fragments) as a source of phenolic compound.

iv

Acknowledgments

I am deeply indebted to my parents and family for their love and support, as well as for having

planted and nurtured the importance of education in my person from a very early age. Without

them this work would not have been possible. I would also like to thank Dr. Maria Auad for her

guide and for giving me the opportunity to pursue a PhD degree. I am also deeply thankful to Dr.

Gisela Buschle-Diller for her kindness and friendship. I want to thank my close friends as well.

I would also like to thank the committee members, Dr. Mario Eden, Dr. Sushil Adhikari, Dr.

Brian Via, and Dr. Mahesh Hosur for their contributions to this project. Thanks to Dr. Allan

David for his willing to be the outside reader for this dissertation.

This work was supported by the NSF under grant # NSF EPS-115882 and by the CREST under

grant # HDR-1137681.

v

Table of Contents

Abstract ......................................................................................................................................... ii

Acknowledgments........................................................................................................................ iv

List of Tables .............................................................................................................................. xii

List of Figures ............................................................................................................................ xiv

Chapter I. Introduction ................................................................................................................. 1

1.1. Overview of the global synthetic polymer industry ............................................................. 1

1.2. Thermosetting plastics and composite materials ................................................................. 3

1.3. Challenges for the future and innovation of the polymeric materials .................................. 5

1.4. Biomass as an alternative to produce green plastics ............................................................ 6

1.5. General pathways towards synthesis of renewable monomers .......................................... 11

1.5.1. Molecular biomass direct from nature ............................................................................ 11

1.5.2. Fermentation or biological transformation ..................................................................... 18

1.5.3. Chemical transformation of natural polymers ................................................................ 20

1.6 Research Objective ............................................................................................................. 26

References ................................................................................................................................. 30

Chapter II. Renewable Thermoset Copolymers from Tung Oil and Natural Terpenes ............ 41

1. Introduction ............................................................................................................................ 44

2. Methods ................................................................................................................................. 44

2.1. Materials ............................................................................................................................ 44

vi

2.2. Polymer preparation via cationic polymerization .............................................................. 44

2.3. Characterization ................................................................................................................. 45

3. Results and Discussion ......................................................................................................... 46

3.1. Cationic Polymerization of vegetable oils ......................................................................... 46

3.2. Characterization of the polymer networks ......................................................................... 48

3.2.1. Thermo-mechanical behavior ......................................................................................... 49

3.3. Theoretical Prediction of Tg based on the Fox-Loshaek Model ........................................ 57

3.4. Conclusions ........................................................................................................................ 61

References ................................................................................................................................. 62

Chapter III. Triglycerides and Phenolic Compounds as Precursors of Bio-Based Thermosetting Epoxy Resins ................................................................................................................ 66

1. Introduction ........................................................................................................................... 66

2. Experimental Section ............................................................................................................ 70

2.1. Materials ............................................................................................................................ 70

2.2. Synthesis Procedures ......................................................................................................... 71

2.2.1. Synthesis of epoxidized triglycerides ............................................................................. 71

2.2.2. Synthesis of polyol .......................................................................................................... 71

2.2.3. Functionalization of phenolic compounds with epoxy groups. Synthesis of triglycidylated

ether of α-resorcylic acid .............................................................................................. 72

2.2.4. Synthesis of bio-based epoxy resins ............................................................................... 72

2.2.5. Preparation of the linseed oil based polyurethane .......................................................... 73

vii

2.3. Characterization of starting and modified materials .......................................................... 74

2.3.1. Fourier transform infrared spectroscopy (FTIR) ............................................................ 74

2.3.2. Epoxide equivalent weight determination ...................................................................... 74

2.3.3. Iodine value determination ............................................................................................. 75

2.3.4. Hydroxyl number determination ..................................................................................... 76

2.4. Characterization of the epoxy networks ............................................................................ 76

2.4.1. Soxhlet extraction ........................................................................................................... 76

2.4.2. Scanning electron microscopy (SEM) ............................................................................ 77

2.4.3. Thermal properties .......................................................................................................... 77

2.4.4. Thermo-mechanical properties of the networks ............................................................. 77

3. Results and Discussion ......................................................................................................... 78

3.1. Synthesis of epoxidized triglycerides ................................................................................ 78

3.2. Synthesis of linseed oil based polyol ................................................................................... 81

3.3. Functionalization of phenolic compounds with epoxy groups .......................................... 84

3.3.1. Synthesis of triglycidylated ether of α-resorcylic acid ................................................... 84

3.3.2. Synthesis of diglycidylated ether of resorcinol ............................................................. 86

3.4. Thermo-mechanical characterization of bio-based epoxy thermosetting resins ................ 87

3.4.1. Polymerization of epoxidized linseed oil through step-growth process ......................... 87

3.5. Preparation of the linseed oil based polyurethane ............................................................. 89

3.6. Polymerization of acrylated soybean oil through free radical mechanism ........................ 89

viii

3.7. Copolymerization of epoxidized linseed oil and epoxidized α-resorcylic acid through chain growth process .............................................................................................................. 92

3.8. Scanning electron microscopy (SEM) .............................................................................. 94

3.9. Thermo-mechanical characterization of the epoxidized linseed oil (ELO) and α-resorcylic acid (ERA) copolymers .................................................................................................. 96

4. Conclusions ......................................................................................................................... 108

References ............................................................................................................................... 110

Chapter IV. Synthesis and Characterization of Interpenetrating Polymer Networks (IPNs) using Biomass Derived Materials .......................................................................................... 116

1. Introduction ......................................................................................................................... 116

2. Experimental Section .......................................................................................................... 121

2.1. Materials .......................................................................................................................... 121

2.2. Synthesis procedures ........................................................................................................ 122

2.2.1. Synthesis of epoxidized triglycerides ........................................................................... 122

2.2.2. General procedure for the glycidylation of α-resorcylic acid ....................................... 122

2.2.3. Synthesis of IPNs ............................................................................................................ 122

2.3. Characterization of materials and networks ....................................................................... 124

2.3.1. Fourier transform infrared spectroscopy (FTIR) ............................................................ 124

2.3.2. Kinetics of gelation ....................................................................................................... 125

2.3.3. Determination and measurement of the extent of the reaction at the gel time ............. 125

2.3.4. Soxhlet extraction ......................................................................................................... 126

ix

2.3.5. Scanning electron microscopy (SEM) .......................................................................... 127

2.3.6. Thermal properties .......................................................................................................... 127

2.3.7. Thermo-mechanical properties of the networks ........................................................... 127

2.3.8. Linear elastic fracture mechanics .................................................................................. 128

3. Results and Discussion ....................................................................................................... 129

3.1. Chemical functionalization of raw materials ................................................................... 129

3.1.1. Synthesis of epoxidized triglycerides ........................................................................... 129

3.1.2. Functionalization of phenolic compounds with epoxy groups. Synthesis of triglycidylated

ether of α-resorcylic acid ........................................................................................... 129

3.2. Characterization of the IPNs ............................................................................................ 129

3.2.1. System A: Epoxidized linseed/acrylated soybean oil IPNs .......................................... 129

3.2.1.1. Scanning electron microscopy ................................................................................... 129

3.2.1.2. Thermo-mechanical characterization of IPNs ........................................................... 132

3.2.2. System B: epoxidized α-resorcylic acid/acrylated oil IPNs ......................................... 138

3.2.2.1. Scanning electron microscopy ................................................................................... 138

3.2.2.2. Kinetics of gelation .................................................................................................... 141

3.2.2.3. Curing behavior of IPNs ............................................................................................ 146

3.2.2.4. Thermo-mechanical characterization ......................................................................... 150

3.2.2.5. Linear elastic fracture mechanics ............................................................................... 151

4. Conclusions ......................................................................................................................... 160

References ............................................................................................................................... 162

x

Chapter V. Fast Pyrolysis Bio-oil as precursor of Thermosetting Epoxy Resins ................... 169

1. Introduction ......................................................................................................................... 169

2. Experimental section ........................................................................................................... 175

2.1. Materials .......................................................................................................................... 175

2.2. Synthesis procedures ........................................................................................................ 176

2.2.1. General method for the glycidylation (functionalization with epoxy groups) of bio-oil........................................................................................................................ 176

2.3. Characterization of starting and modified materials ........................................................ 177

2.3.1. Gas chromatography –Mass spectrometry (GC-MS) ................................................... 177

2.3.2. Hydroxyl group analysis through 31P-NMR spectroscopy ........................................... 177

2.3.3. Fourier Transform Infrared Spectroscopy (FTIR) ........................................................ 178

2.3.4. Epoxide equivalent weight determination .................................................................... 178

2.4. General procedure for the formulation of crosslinked epoxy networks .......................... 179

2.5. Characterization of epoxy networks ................................................................................ 179

2.5.1. Sohxlet extraction ......................................................................................................... 179

2.5.2. Scanning electron microscopy (SEM) .......................................................................... 180

2.5.3. Thermo-mechanical properties of the networks ........................................................... 180

3. Results and Discussion ....................................................................................................... 181

3.1. Gas chromatography – Mass spectroscopy (GC-MS) ....................................................... 181

3.2. Hydroxyl group analysis through 31P-NMR spectroscopy ............................................... 185

xi

3.3. General method for the glycidylation (functionalization with epoxy groups) of bio-oil ....................................................................................................................... 187

3.3.1. Glycidylation of the α-resorcylic acid .......................................................................... 187

3.3.2. Glycidylation of bio-oil................................................................................................... 187

3.4. Fourier transform infrared spectroscopy (FTIR) ............................................................. 189

4. Characterization .................................................................................................................. 190

4.1. Soxhlet extraction ............................................................................................................ 190

4.2. Scanning electron microscopy (SEM) ............................................................................. 192

4.3. Thermo-mechanical characterization of bio-oil based epoxy thermosetting resin ............ 194

4. Conclusions ........................................................................................................................... 200

References ............................................................................................................................... 201

General conclusions ................................................................................................................ 206

Future work ............................................................................................................................. 211

xii

List of Tables

Table 1.1 Global plastic consumption for the year 2012 .............................................................. 2

Table 1.2 Annual consumption (2012) of major thermosets (1000 tons and percentage). ........... 3

Table 1.3 Average biomass content of the bio-based polymers currently developed. ................. 7

Table 1.4 Classification of mayor molecular biomass ……………………………...……..…..10

Table 1.5 Names and double bonds positions of common fatty acids. ................................. …..13

Table 1.6 Platform molecules identified by the United States Department of Energy. .............. 19

Table 1.7 Chemical classification of bio-oil components ........................................................... 23

Table 2.1 Properties of the tung oil-limonene copolymers ......................................................... 52

Table 2.2 Properties of the tung oil-myrcene copolymers .......................................................... 54

Table 2.3 Properties of the tung oil-styrene copolymers ............................................................ 56

Table 3.1 Properties of some common oils. ................................................................................ 79

Table 3.2 Properties of the resulting linseed oil based polyol. ................................................... 82

Table 3.3 Properties of the acrylated soybean oil polymers. ...................................................... 90

Table 3.4 Mechanical properties of the triglycidylated ether of α-RA and epoxidized linseed oil epoxy resins. ............................................................................................................. 97

Table 3.5 Mechanical properties of the triglycidylated ether of α-RA and resin EPON 828 epoxy resins. ................................................................................................................ 104

Table 3.6 Thermo-mechanical properties of the triglycidylated ether of α-RA and diglycidylated ether of resorcinol copolymers. ........................................................... 105

xiii

Table 4.1 Evolution of the storage modulus (E’) and glass transition temperature as a function of epoxy/acrylate weight proportion.. .......................................................................... 132

Table 4.2 Extent of the reaction at the gel point for IPNs based on epoxidized α-resorcylic acid and acrylated soybean oil.. ........................................................................................... 145

Table 4.3 Onset and maximum of curing peak temperatures for the IPNs based on epoxidized α- resorcylic acid and acrylated soybean oil.. .............................................................. 146

Table 4.4 Mechanical Properties of the IPNs based on epoxidized α-resorcylic acid and acrylated soybean oil.. ................................................................................................................. 153

Table 4.5 Experimental KIc and GIc values. .............................................................................. 156

Table 5.1 Major components found in pyrolysis oil. ................................................................ 173

Table 5.2 Major pyrolysis decomposition products identified by GS/MS. .............................. 183

Table 5.3 Fractions of phenolic compounds found in bio-oil. .................................................. 184

Table 5.4 Hydroxyl number (OHN) of pyrolysis oil determined by quantitative31P-NMR…………….. ………………………………………............. …186

Table 5.5 Thermo-mechanical properties of epoxidized bio-oil and the triglycidylated ether of α-resorcylic acid (epoxidized α-resorcylic) resins. .................................................. 196

Table 5.6 Mechanical properties of the epoxidized bio-oil, triglycidylated α-RA and Super Sap 100 epoxy. .................................................................................................................... 199

xiv

List of Figures

Figure 1.1 Global production and demand of plastics by geographical area by percentage (2012) ....................................................................................................... 2

Figure 1.2 Plastics classification ................................................................................................... 5

Figure 1.3 Trend of publications for polymers from renewable resources ................................... 9

Figure 1.4 Major pathways to produce monomers (or precursors) from biomass. ..................... 11

Figure 1.5 Structure of a triglyceride. Where R1, R2, and R3 are fatty acid chains .................... 12

Figure 1.6 Chemical transformation of biomass. ........................................................................ 21

Figure 2.1 Cationic polymerization of tung oil with limonene, myrcene, and linalool .............. 47

Figure 2.2 Temperature dependence of the (2.2.a) storage modulus (E’) for the copolymers tung oil-limonene. (2.2.b) Loss factor (tan δ) as a function of the temperature for the tung oil- limonene copolymers. .................................................................................................. 50

Figure 2.3 (2.3.a) Glass transition temperature of tung oil-limonene copolymers as a function of

the composition: (▲) values measured with DMA, (♦) linear copolymer contribution, (■) glass transition temperature of the copolymer. (2.3.b) Glass transition temperature of tung oil-mycene copolymers as a function of the composition: (▲) values measured

with DMA, (♦) linear copolymer contribution, (■) glass transition temperature of the copolymer. .................................................................................................................... 58

Figure 3.1 Epoxidation of triglycerides ...................................................................................... 79

Figure 3.2 FTIR spectrum of epoxidized linseed oil .................................................................. 81

Figure 3.3 Methanolysis of epoxidized linseed oil ..................................................................... 81

xv

Figure 3.4 FTIR of hydroxylated linseed oil .............................................................................. 83

Figure 3.5 Glycidylation reaction of α-resorcylic acid with epichlorohydrin ............................ 84

Figure 3.6 FTIR spectra of α-resorcylic acid and epoxidized α-resorcylic acid ........................ 85

Figure 3.7 Glycidylation reaction of resorcinol acid with epichlorohydrin................................ 86

Figure 3.8 FTIR spectra of resorcinol and epoxidized resorcinol .............................................. 87

Figure 3.9 Storage modulus and tan δ of epoxidized linseed oil as a function epoxy/amine ratio ........................................................................................... 88

Figure 3.10 Anionic polymerization of epoxy monomers. ......................................................... 93

Figure 3.11 SEM micrographs of the fracture surface of 100 % α-RA (3.11.a), 80 % α-RA – 20 %

ELO (3.11.b), 70 % α-RA – 30 % ELO (3.11.c), and 100 % ELO (3.11.d). ..................... 94

Figure 3.12 Temperature dependence of the storage modulus (3.12.a) and loss factor or tan δ (3.12.b) as a function of the composition of the epoxy resins ...................................... 96

Figure 3.13 Relationship between 1/Tg and relative epox. α-resorcylic acid weight fraction .............................................................................................................. 99

Figure 3.14 SEM micrographs of the fracture surface of 100 wt% α-RA (3.14.a), 75 wt%

α-RA – 25 wt% EPON 828 (3.14.b), 50 wt% α-RA – 50 wt% EPON 828 (3.14.c), and

100 wt% EPON 828 (3.14.d). .................................................................................... 102

Figure 3.15 Temperature dependence of the storage modulus (3.15.a) and loss factor or tan δ (3.15.b) as a function of the composition of the resorcinol weight content for the

resorcinol – α-resorcylic acid epoxy resins ............................................................. 106

Figure 4.1 Generic schemes of the IPNs containing epoxy and acrylate networks. ................ 120

xvi

Figure 4.2 Chemical structure of the acrylated soybean oil. .................................................... 122

Figure 4.3 SEM micrographs of the fracture surface of 100 % epoxidized linseed oil (4.3.a), IPN 80% epoxy - 20% acrylate (4.3.b), IPN 60% epoxy - 40% acrylate (4.3.c), IPN 40% epoxy – 60%acrylate (4.3.d), IPN 20% epoxy-80% acrylate (4.3.e), 100% acrylated soybean oil (4.3.f) ............................................................................. 130

Figure 4.4 Crosslinking densities as a function of the epoxy weight fraction. ......................... 134

Figure 4.5. SEM micrographs of the fracture surface of 100 wt% epoxidized α-resorcylic acid (4.5.a), IPN 75 wt% epoxy – 25 wt% acrylate (4.5.b), IPN 50 wt% epoxy – 50 wt% acrylate (4.5.c), IPN 25 wt% epoxy – 75 wt% acrylate (4.5.d), 100 wt% acrylated soybean oil (4.5.e)…………………………………………………………………....139

Figure 4.6. Gel times at 65°C for the studied IPN systems based on epoxidized α-resorcylic acid and acrylated soybean oil.. .................................................................................. 142

Figure 4.7. Extent of conversion of stoichiometric mixture of epoxidized α-resorcylic acid and multifunctional amine (Jeffamine T-403). ................................................................. 144

Figure 4.8 Heats of polymerization (4.8.a) and conversions (4.8.b) of the IPNs based on

epoxidized α-resorcylic acid and acrylated soybean ................................................. 147

Figure 4.9. Dynamic mechanical response of IPNs based on epoxidized α-resorcylic acid and acrylated soybean oil.................................................................................................. 150

Figure 4.10. GIc versus Mc1/2 for the studied IPNs. ................................................................... 159

Figure 5.1. Monomeric phenylpropane unit structures.. ........................................................... 171

Figure 5.2. Total ion GC/MS chromatogram of fast pyrolysis bio-oil.. ................................... 182

xvii

Figure 5.3. Glycidylation of bio-oil with epichlorohydrin in alkaline medium. ....................... 188

Figure 5.4. FTIR spectra of raw bio-oil, modified bio-oil, and epichlorohydrin. ..................... 190

Figure 5.5. Mass loss (wt%) of bio-oil / α-resorcylic acid epoxy resins. ................................. 191

Figure 5.6. SEM micrographs of the 100 wt% epoxidized bio-oil (5.6.a), 50 wt% epoxidized

biooil – 50 wt% epoxidized α-RA (5.6.b), and 100 wt% epoxidized α-RA (5.6.c) resins.. ........................................................................................................................ 193

Figure 5.7. Temperature dependence of the storage modulus (5.7.a) and loss factor or tan δ (5.7.b) as a function of the composition of the epoxy resins. .................................... 194

Figure 5.8. Relationship between 1/Tg and relative epoxy bio-oil weight content (Wbio-oil). ... 198

1

CHAPTER I

Introduction

Plastics, or synthetic polymers, are one of the first primary man-made materials and they have

been applied in a variety of applications. Plastics are made of raw chemicals that mainly include

petroleum based products of its refinery process, natural gas, carbon, and other natural

compounds. Their impact in the modern society can be traced to research-related developments,

the discovery of new and innovative materials able to fulfill the needs of the industrial sectors,

and the gradual substitution of heavier and less versatile materials, such as metals, glass,

ceramics, etc. Starting in the 1950s with the development of the macromolecular theory

developed by scientists such as Herman Staudinger, H.W. Carothers, Paul Flory, etc., the

worldwide production of plastic has shown an important increase. In the following section, an

overview of the global plastic industry is given. The goal of the figures and details is to help

clarify the real perspective about the current state of the plastic industry.

1. Overview of the global synthetic polymer industry.

For the year 2012, the worldwide plastic consumption was of the order of 270 million tons, with

a turnover of around $1,100 billion or more (Table 1.1). Over the past years, the global plastic

consumption has grown consistently, showing an average annual rate that is consistent with this

trend. For the next years, this growth is estimated to have an annual increase of approximately

5.5%, reaching an estimate average value of 400 million tons by the year 2020 [1].

2

Other

common-

wealth

3% Japan

5%

Latin

America

5%Middle

East, And

Africa

7%

Rest of

Asia

16%Europe

20%

China

24%

NAFTA

20%

Demand

Table 1.1. Global plastic consumption for the year 2012

Million tons %

Thermoplastics 215 80

Thermosets 30 11

Composites 10 4

Others 15 5

Total 270 100

Source: Biron, M. Thermosets and Composites, 2014

Consumptions and growth rates widely vary according to the considered area. China and other

Asian nations are leaders in commodity plastic consumption, while North America and Western

Europe are leaders for engineering and specialty plastics (Figure 1.1).

Figure 1.1. Global production and demand of plastics by geographical area by percentage, 2012.

Source: The European House-Ambrosetti re-elaboration of PlasticsEurope data, 2013 [2].

CSI

3%

Japan

5%

Latin

America

5% Middle

East, And

Africa

7%

Rest of

Asia

16%

NAFTA

20%

Europe

21%

China

23%

Production

3

1.2. Thermosetting plastics and composite materials

Thermosetting materials are polymeric materials that cure irreversibly. They are generally

stronger than thermoplastic materials due to their three-dimensional network of bonds (cross-

linking), and are also better suited to high-temperature applications up to the decomposition

temperature, however, they are considerably brittle. Since their shape is permanent, they tend not

to be recyclable as a source for newly made plastic. According to the review “Global

Thermosetting Plastics Market - Segmented by Type, Industry and Geography - Trends and

Forecasts (2015-2020)” [3], the global thermosetting plastics production was estimated to be

33.74 million tons in 2014, and is expected to reach 41.96 million tons by 2020; showing a

compound annual growth rate of 3.7%. Asia-Pacific currently dominates the market for

thermosetting plastics followed by North America and Europe. This increase is primarily

supported by the growth in the use of unsaturated polyurethanes, phenol-formaldehyde and

amino/epoxy resins, to supply the needs of an increasing number of markets from construction,

adhesives & coatings, electrical & electronics, automotive & transportation and a number of

other segments or fields (Table 1.2). Table 1.2 shows the annual consumption of thermosetting

materials in the world market.

Table 1.2. Annual Consumption of Major Thermosets (1000 tons and percentage), 2012.

USA Europe Asia World

kt % kt % kt % kt %

Polyurethanes 2500 36 2000 31 7000 61 14000 47

Unsaturated Polyesters 1500 21 1500 23 1700 15 5500 19

Phenolic Resins 1000 14 800 12 800 7 3000 10

Amino Resins 1300 18 1500 23 1300 12 5300 18

Epoxies 400 6 400 6 400 4 1500 5

Others 300 5 300 5 100 1 700 1

Total for major thermosets 7000 100 6500 100 11300 100 30000 100

Total plastic consumption 270000

Major thermoset share of the plastic total

11%

Source: Biron, M. Thermosets and Composites, 2014

4

The mentioned thermosets represent 11% of global plastic consumption. Likewise, interest in

composite materials has constantly grown in the last decades, striving for obtaining new

composites with superior mechanical strength, better thermal and electrical properties, and high

resistance to external factors while reducing the weight [4]. In most of the structural and

engineering applications, composites generally comprise a bulk phase (known as the matrix) and

a reinforcing phase (the reinforcement). The matrix provides the composite with its shape,

surface appearance, environmental tolerance and durability by integrally binding the

reinforcement; being the last one the component carrying most of the structural loads [5]. In the

case of polymer composites, matrix materials can be either a thermosets or thermoplastics.

Common thermosetting polymers employed in composite materials include polyurethanes, epoxy

resins and polyesters. Generally, their mechanical performance requires them to have modulus

values in the range of 0.025 to 6.0 GPa [6]. It is due to this direct link between the composites

and the polymeric materials that the composites markets are highly influenced by the changes

and innovations in the macromolecular science. Based on these figures and future projections, it

is clear that the polymer industry plays a prominent role and is essential to a variety of industries.

Nowadays, most commercially available polymeric materials are derived from non-renewable

resources and account for approximately 7% of global oil and gas use [7]. It is the combination

of environmental awareness, the depletion of fossil feedstocks and fluctuations in their prices, as

well as financial drivers and global policies that have catapulted the utilization of renewable

resources as raw materials for the production of polymeric materials; and the generation of

biomass-derived materials with properties similar to those shown by their commercial

counterparts remains as a challenge in the academic and industrial sectors.

5

1.3. Challenges for the future and innovation of the polymeric materials.

The polymer industry traditionally has been an area of high level of innovation, leading to major

investment in research and development programs, in response to modern day needs, such as low

cost, high performance, recycling, light weight, and low environmental impact. In the future,

green composite materials and bioplastics will be two further important sectors. Bioplastic is a

term that has been coined as a term for describing plastic derived completely, or partially, from

biological and renewable raw materials instead of fossil fuels (carbon, crude oil and natural gas)

[2]. The term bioplastic depicts a chemical process during which microorganisms that are

available in the environment convert materials into natural substances such as water, carbon

dioxide, and compost. The process of biodegradation is dependent on factors such as

environmental conditions, material, and application. To illustrate the distinction between these

two terms, Figure 1.2 shows the European Bioplastics two-axis model, that encompasses all

plastics types and combinations [8].

Figure 1.2. Plastics classification

Source: http://www.european-bioplastics.org/bioplastics/

Bio-based

Fossil-based

Non

Biodegradable Biodegradable

Bioplastics

e.g. PLA, PHA

PBS, Starch blends

Bioplastics

e.g. PBAT, PCL

Bioplastics

e.g. bio-based PE,

PET, PA, PTT

Conventional

Plastics

e.g. PE, PP, PET

6

The horizontal axis shows the biodegradability of the plastics, whereas the vertical axis depicts

the source from which the plastics are produced. The plastics are therefore divided into four

different groups, which include plastics which are not biodegradable and made from

petrochemical resources, biodegradable plastics from renewable resources, biodegradable

plastics from fossil resources, and non-biodegradable plastics from renewable resources [9].

In 2011, the global production of bioplastics was 1.2 million tons, and it is expected to increase

up to 5.8 million tons in 2016. Asia (34.6%), Latin America (32.8%), and Europe (18.5%) are

the worldwide leaders of bioplastics. It is clear that fossil sources are still required for the

production of major part of the polymers, as only 5% of chemicals are currently produced from

renewable feedstocks [10]. However, the application of natural chemicals as feedstock for the

manufacture of polymers is steadily increasing. Taking into account the great diversity of

biomass; consisting of polysaccharides, aromatic compounds, triglycerides, proteins, etc., a great

number of alternative monomers and novel polymer are potentially accessible [10].

1.4. Biomass as an alternative to produce “green materials”

Biomass is considered the most abundant natural resource with an annual production of 1.3

billion dry tons in the U.S.A. [11]. Biomass is any organic matter that is renewable over time,

and in the context of energy it is often used to mean plant-based materials such as agricultural

residues, forestry wastes, and energy crops [12]. The world energy crisis has led to the

development of programs that have provoked extensive research programs on biomass

conversion. The U.S. Department of Energy (DOE) and the U.S. Department of Agriculture

(USDA) have prioritized the development of bioenergy and bioproducts, and they have a goal to

7

produce 18% of the current U.S. chemical commodities from biomass by 2020, and 25% by 2030

[11,13]. Increased research on the utilization of lignocellulosic biomass to produce bio-based

resins has also been the subject of industrial entrepreneurships. On 2012, the nova-Institute in

Germany released the results of a market study named “Bio-based polymers in the world -

production, capacities, and applications: status quo and trends towards 2020” [14]. This market

report is continually updated, and it is considered the most comprehensive study in this field ever

made. It is the first time that a study has looked at every kind of bio-based polymer produced by

247 companies around the world. One of the most important sets of information is given in the

following Table (1.3), which gives an overview of the average biomass content of the bio-based

polymers currently developed by some industries at a global level.

Table 1.3. Average biomass content of the bio-based polymers currently developed.

Bio-based polymers Average biomass content

of the polymer

Producing companies

until 2020

Cellulose Acetate 50% 9

Polyamide Rising to 60% 14

Polybutylene Adipate Terephthalate Rising to 50% 3

Polybutylene Succinate Rising to 80% 11

Polyethylene 100% 3

Polyethylene Terephthalate 30 to 35% 4

Polyhydroxyl Alkanoates 100% 14

Polylactic Acid 100% 27

Polypropylene 100% 1

Polyvinyl Chloride 43% 2

Polyurethane 30% 10

Starch Blends 40% 19

Source: Bio-based polymers in the world - production, capacities, and applications: status quo and trends towards

2020

8

The report shows that the production capacity of bio-based polymers will triple from 3.5 million

tons in 2011 to nearly 12 million tons by 2020. Mayor players in bulk biopolymers production

include companies like Metabolix, Dow Chemicals, Novamont, Cereplast, Teijin, Natureworks,

Hisun, Tianan, Plantic, Innovia, Proter & Gamble, Kaneka, and Arkema [15]. In the case of

bioplastics, some mayor companies include BASF, DuPont and Braskem [16].

According to this results, it is clear that they development of green polymers is growing, and that

it is supported not only by the industrial sectors, but also by governmental entities and agencies.

In the case of the academic field, the worldwide interest in bio-based polymers has increased

progressively in the past years, as the advances in technology have generated an accumulated

experience, allowing the development of new routes to produce bioplastics to reduce the

dependency on fossil fuels. As an example of the evolution of this field, a search of the Web of

Science database using the words “polymers from renewable resources” identified a total of

more than 1500 scientific articles currently available on the subject. Figure 1.3 shows that there

has been an exponential growth in the last 20 years, with approximately 86% of them published

in the last decade, clearly evidencing the increasing interest in this promising field. Concerning

the countries, the USA leads the trend in publications in the bioplastic development, ranking first

with approx. 23 % of the more than 1500 papers matching the online search, followed by

countries like Spain, Japan, and China.

9

Figure 1.3. Trend of publications for polymers from renewable resources

Source: Thomson Reuters, May 2016

Within the academic field, it has been a general goal to develop renewable polymers by replacing

and resembling the existing polymeric materials derived from petroleum chemicals. Over the last

decade, extensive and comprehensive reviews have been published. Some of these studies

include the review of the catalytic reactions that help to transform carbohydrates, vegetable oils,

animal fats, and terpenes into valuable or potentially valuable chemicals [17,18]. Also, current

methods and future possibilities for obtaining transportation fuels from biomass are extensively

discussed by Huber, Iborra, and Corma [19]. It is important to highlight that some efforts have

been developed to compare materials based on renewable resources and petrochemistry-based

materials with respect to their ecological potential [20]. Likewise, scientific findings concerning

the recycling of bioplastics, their blends and thermoplastic biocomposites; have been reviewed

by Soroudi and Jakubowicz [21]. According to their natural molecular biomass origins, some of

the monomers that have been studied as substitutes of fossil-based materials can be classified

into four different categories based on hydrogen, carbon, and oxygen compositions, which are

detailed in Table 1.4.

0

25

50

75

100

125

150

175

200

225

19

86

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00

20

01

20

02

20

03

20

04

20

05

20

06

20

07

20

08

20

09

20

10

20

11

20

12

20

13

20

14

20

15

Published items each year

10

Table 1.4. Classification of mayor molecular biomass

Natural Molecular Biomass

Oxygen-rich biomass

Hydrocarbon-rich

biomass

Hydrocarbon

biomass

Non-hydrocarbon

biomass

Carboxylic Acids Vegetable Oils Ethene Carbon Dioxide

Polyols Fatty Acids Propene Carbon Monoxide

Dianhydroalditols Terpenes Isoprene

Furans Terpenoids Butene

Resin Acids

Source: K. Yao, C. Tang, 46 (2013) 1689–17

As a general trend, there are three major pathways to produce monomers (or their precursors)

from natural resources, which include and molecular biomass direct from nature, fermentation or

biological transformation, and chemical transformation of natural polymers [22]. Figure 1.4

depicts a scheme of the three approaches to obtain chemicals for the polymer industry. In the

following section, the molecular biomass direct from nature and chemical transformations are

discussed more in depth than fermentation, which is briefly described, due to their significant

role in the production of the monomers and precursors studied in this work.

11

Figure 1.4. Major pathways to produce monomers (or their precursors) from biomass

1.5. General pathways towards synthesis of renewable monomers.

1.5.1. Molecular biomass direct from nature.

Different from the compounds obtained through processes such as fermentation or chemical /

thermal degradation, there is a considerable amount of chemicals or precursors that can be

directly obtained from nature. This pathway already stands as one of the cornerstones for using

renewable resources, both on large (pulp, paper and starch industry) and on small scale

(production of fine chemicals). Within this category, a rich pallet of monomers structures such as

terpenes, rosin, sugars, polycarboxylic acids and aminoacids, furans, and vegetable oils can be

exploited as a source of materials for macromolecules industry. Since a considerable part of the

experimental work described in this manuscript deals with the use of vegetable oils as a platform

for polymeric materials, the next paragraphs review basic information regarding the reasons why

Biomass

Molecular biomass direct from nature

Vegetable oils, carbohydrates, phenolic acids,

terpenes, proteins

Fermentation or biological

transformation

Etanol, sorbitol, glucose, diacids,

glycerol

Chemical Transformation

Thermochemical

degradation

Chemical treatments

12

these compounds have awakened great interest as sustainable and green materials. Also, other

types of chemical precursors obtained from biomass are considered and reviewed.

Oil based bio-products

In the year 2000, the production of oils and fats amounted to 17.4 kg per year per capita and a

growth rate of 3.3 % per year was estimated. The total worldwide yearly production of

renewable materials was over 110 million tons in 2002, and vegetal oils constitutes the 80 % of

this production (the remaining 20 % being animal fats). Only 15.6 million tons (around 15 per

cent) of these raw materials are used as precursors to the synthesis of new chemical commodities

and materials [23]. Nowadays, their main application lies in the field of biodiesel, which can be

used as an alternative engine fuel. However, considerable work on polymers derived from plant

oils have been performed in the last few years [24,25]. Vegetable oils are hydrocarbon rich

compounds that are liquid at room temperature and mostly composed of triglycerides. These

chemical compounds have a three-armed star structure in which three fatty acids are joined at a

glycerol junction (Figure 1.5), and these fatty acids contribute from 94 – 96% of the total weight

of one molecule of triglyceride [25]. The chain length of these fatty acids can vary from 14 to 22

carbons and contain 0 – 5 double bonds situated at different positions along the chain and in

conjugated or unconjugated sequences [23].

O

O

O R1

O

O

R3

O R2

Figure 1.5. Structure of a triglyceride. Where R1, R2, and R3 are fatty acid chains.

13

Table 1.5 collects the 16 most important oils with their specific chemical structures. All of the

mentioned oils do not include conjugated double bonds in their structures (polymers, monomers,

composites).

Table 1.5. Names and double bonds positions of common fatty acids.

Chain length : number of double bonds

Systematic name Trivial name Double bond position

12:0 Dodecanoic Lauric -

14:0 Tetradecanoic Mystiric -

16:0 Hexadecanoic Palmitic -

18:0 Octadecanoic Stearic -

18:1 9-Octadecanoic Oleic 9

18:2 9,12-Octadecanoic Linoleic 9;12

18:3 6,9,12-Octadecanoic γ-linolenic 6,9,12

18:3 9,12,15-Octadecatrienoic α-linolenic 9,12,15

20:0 Eicosanoic Arachidic -

20:1 Eicosanoeic - 9

20:4 Eicosatetraenoic Arachidonic 5,8,11,14

20:5 Eicosapentaenoic EPA 5,8,11,14,17

22:0 Docosanoic - -

22:1 Docosenoic Euric 13

22:5 Docopentanoic DPA 7,10,13,16,19

22:6 Docosahexanoic DHA 4,7,10,13,16,19 Source: A. Gandini, M.N. Belgacem, Monomers, Polymers and Composites, Elsevier, UK, 2008

As shown, some fatty acids are saturated (having no double bonds). On the other hand,

unsaturated fatty acids can have one or more than one double bond in their structure. It is

important to mention that, additionally to the above described oils, some naturally fatty acids

have different organic functional groups attached to the hydrocarbon backbone, such as

hydroxyl, epoxy, and triple bonds. This structural diversity, as well as the number of double

bonds and their position within the aliphatic chain, strongly affects the chemical and physical

properties of the oils [26]. When vegetable oils are used as a source of monomers to produce

polymeric materials, the double bonds, allylic positions, ester groups and other functional groups

are the main targets of chemical reactions [27].

14

During the last decade, a variety of oil-based polymeric systems have been developed and

studied, mainly address to replace or diminish the use of existing petroleum-based resins.

Unmodified plant oils have been used to prepare renewable polymers by thermal [28], and

cationic [29] polymerization methods. Copolymerization is a common practice in polymer

science, and it is used to modify the properties of manufactured plastics to meet specific needs of

the market and final application. Plant oils have not been an exception to this approach, and

several investigations have been published concerning the copolymerization of unmodified oils

with common crosslinking agents such as styrene, divinylbenzene, norbornadiene, and

dicyclopentadiene [30,31]. Less common commercial agents such as 4-vinylphenyl boronic acid

and tris(4-vinylphenyl)boroxine have been also studied [32]. By varying the structure and

stoichiometry of the alkene comonomers and the vegetable oils, polymers ranging from soft to

hard rubbers can be obtained. However, this approach generates partially bio-based resins, and

for obtaining more rigid and crosslinked materials; higher amounts of these expensive

commercial comonomers are required [33].

Due the richness of the hydrocarbon chemistry, several oil-based polymeric materials can be

synthesized by introducing new functional groups through reactions or chemical modifications.

Most of the natural unsaturated oils contain cis-configured alkenes allowing, in principle, the

application of well-known reaction of petrochemical alkenes. Remarkably, only very few

reactions across the double bond of unsaturated oils are currently applied in chemical industry.

The key concept behind chemical modification is to reach a higher level of molecular weight and

crosslinking density, factors that are known to deeply impact stiffness in the polymer network

[34]. This can be accomplished by several pathways. From natural triglycerides, it is possible to

15

attach maleates [35], epoxy [36], acrylate [37], allylic double bonds [38], amine [39,40], and

hydroxyl [41] functionalities. Such transformations make the triglyceride capable of reaction via

ring opening, free radial or polycondensation reactions, which are common mechanisms to

polymerize commercial monomers. These approaches involve the modification of triglycerides.

A different method of chemical modification is to convert the triglyceride into monoglycerides

through glycerolysis or amidation reactions. An extension of this method implies the

functionalization of the unsaturated sites, and then a reduction of the triglyceride into

monoglycerides. These derivatives have found applications as coatings [34].

Recently, relatively more advanced polymerization methods such as ring opening metathesis

(ROMP) of modified oils containing norbornene moiesties have been employed to prepare

macromolecular materials out of vegetable oils [42]. Likewise, in the composites field, glass

fiber bio-based composites have been prepared by the same technique using a vegetable oil

derivative bearing an unsaturated bicyclic moiety and dicyclopentadiene [43]. A thorough and

vast review about possible reactions using oil and fats as substrates is given by Biermann et al.

[44]. From the literature review, it was been very clear that all of the above mentioned

approaches lead to polymeric materials which are not fully, but partially, derived from biomass.

Evidently, there is a need to provide materials with higher contributions of biomass derived

compounds per weight basis.

16

Terpenes

In contrast to common petroleum derived monomers for the plastic industry, such as ethylene

with a molecular weight of 28 g/mole; triglycerides obtained from plant oils can be considered as

organic “macromonomers”, with molecular weights approaching 880 g/mol. As previously

stated, triglycerides are composed of chains with a number of carbon atoms ranging to 14 to 22

attached to a glycerol core, and these chains structurally mimic the behavior of olefins such as

ethylene, propylene, and isoprene. As one of the major classes of petroleum chemicals,

cycloaliphatic, and aromatic compounds such as styrene, divinyl benzene, etc. confer rigidity and

hydrophobicity to the polymers derived from them [45]. Besides from biomass refinery,

compounds structurally resembling cycloaliphatic and aromatic compounds can be directly

obtained from biomass. Within this category, it is of worth to mention the possibility of

exploring terpenes, terpenoids, and rosin as a class of important bearing cyclic and aromatic

structures. A vast review about sources, structure and reactivity of rosin is given by Maiti, Ray,

and Kundu [46].

Terpenes are unsaturated aliphatic structures, predominantly derived from turpentine, the volatile

fraction of resins exuded from conifers. Turpedine production amounts to approx. 350 000

tons/year. Most terpenes have a cycloaliphatic structure with isoprene (C5H8) as a carbon

skeleton building unit, and they can be classified according to the number of isoprene units [23].

Terpenoids can be considered as modified terpenes, wherein methyl groups have been moved o

removed, or oxygen atoms added [45]. Terpenes are a good starting material for the synthesis of

fine chemicals due to their similar carbon skeleton. Monoterpenes such as Pinene and Limonene

(which represent more than 90 % of the orange peel oil) are abundant and inexpensive and have

17

been extensively used as chemicals for fragrances, flavors, pharmaceuticals, and solvents [47].

Concerning their use as starters of polymeric materials, very few of these compounds have been

the subject of studies. In Chapter II, cationic copolymerization of tung oil (a natural vegetable

oil) with three different terpenes; such as limonene, myrcene and linalool as reactive

comonomers is studied and discussed. No other study has been reported on copolymerizing plant

oils with terpenes.

Sugars and starch bioproducts

As previously presented, the diversity and the worldwide production capabilities for renewable

and sustainable biomass are enormous. These two factors, along with governmental legislations

mandating increases in the gross domestic and energy and chemical production from renewable

resources, are contributing to the intensified interest in the development of technology and

processes for biomass valorization. From long ago, the chemical literature on this topic has

mostly focus on the use of biopolymers, primarily on products obtained from cellulose [48]. The

use of lignin, plant oils, terpenes, sugars, polyphenols and other biomass derived compounds as

chemical intermediate platforms to develop polymeric materials with potential structural and

composite applications stands up as a field in which considerable scientific research can be

developed.

Concerning sugar and starch bioproducts, novel bio-based high functionality epoxy resins have

been synthetized by the epoxidation of sucrose esters resins of vegetable oils fatty acids [49].

The same research group extended their work using dipentaerythritol, and tripentaerythrytol as

core polyols that were also substituted with epoxidized soybean oil fatty acids [50]. Relatively

18

recently, Chrysanthos, Galy and Pascault [51], developed a bio-based epoxy network from

isosorbide diglycidyl ether via epichlorohydrin that mimics the structure of the well-known

diglycidyl ether of bisphenol A (DGEBA). An overview of advances in sugar-based polymers is

given by Feng et al. [52]. Starch has also impacted the polymers technology. In starch-based

bioplastics starch is fully utilized with a yield very close to 100 %, whereas in starch-derived

bioplastics (synthesized from monomers resulting from the fermentation of glucose syrup). The

yield is generally less than 45 %. Polymers based on starch have a wide range of final properties,

being as flexible as polyethylene or as rigid as polystyrene [53].

Cellulose derivatives, fibers, and plastics

In the case of cellulose, making pure cellulose bioplastics from bio-sources is still highly

challenging due to the fact that it has a strong and highly structured network that cannot be melt

or dissolved by standard process such as thermoforming or dissolution [54]. Due to this

limitation, cellulose has been used as filler for biodegradable plastics and composites. Goffin et

al. [55] incorporated cellulose nanowhiskers into polylactide based composites. Likewise,

recycled cellulose fibers have been combined with bacterial polyester, PHBV, by melt mixing

technique [56].

1.5.2. Fermentation or Biological Transformation

In these processes biological agents take over the task of facilitating the synthesis of products.

This is the most-developed pathway to transform biomass into monomers via fermentation of

carbohydrates [22]. An extensive review considering the use of carbohydrates as a source of

19

compounds such as lactic acid, acids and alcohols by means of fermentations and

biotechnological conversions is given by Lichtenthaler [57].

According to the biorefinery scheme described in the biomass program of the US Department of

Energy [58], part of the biomass is converted to fuels via thermal treatments (pyrolysis and

gasification), and the other is converted by fermentation or chemo-catalytic routes. Table 1.6

provides the 12 platform chemicals that have been identified by the US Department of Energy as

starting materials to produce chemicals and polymeric materials via catalytic routes [58]. The

fermentation processes employed for the synthesis of some of these chemicals are pair with the

advances in genomics and bioprocess engineering, where new genetically modified bacteria or

yeasts continuously improve their production.

Table 1.6. Platform molecules identified by the United States Department of Energy.

Aspartic acid 1,4-Diacids (succinic, fumaric, malic)

Glutamic acid 3-hydroxypropionic acid

Levulinic acid Glycerol

2-hydroxypropionic acid Sorbitol

2,5-Furan dicarboxylic acid Xylitol / arabitol

Glucaric acid 1,4-Diacids (succinic, fumaric, malic)

Itaconic acid 3-hydroxypropionic acid

Source: P. Gallezot, Catal. Today. 121 (2007) 76–91.

From Table 1.6, glycerol needs to be highlighted as a valuable carbon source for the production

of fuels and chemicals. It is a byproduct generated during both bioethanol and biodiesel

production processes, and the massive growth of these two industrial has led to a decrease in

crude glycerol prices [59]. By means of anaerobic fermentation, the use of glycerol permits the

20

co-production of ethanol and formic acid (or ethanol and hydrogen), 1,3-propanediol, 1,2-

propanediol, succinic acid, propionic acid, ethanol and butanol [60].

1.5.3 Chemical transformation of natural polymers.

This pathway involves degradation and transformation of natural materials by means of two

main approaches, the chemical treatments and the thermochemical degradations, which are

shown in Figure 1.6. The main challenge for biorefinery is the efficient, economic, and

sustainable design of pretreatment processes allowing breaking and opening the complex

lignocellulosic structure to facilitate the obtention of chemicals and fuels [61]. These chemical

treatments have gained attention as promising methods to improve the biodegradability of

cellulose, by removing the lignin and hemicellulose fractions. Common chemical treatments

require agents such as oxidizing agents, alkali, acids, organic solvents, etc. able to break bonds,

and thus lowering the degree of polymerization, or by extracting certain compounds of interest,

such as oligomers, alcohols, glucose, xylose, etc. [62,63].

In spite the progress in the chemical treatments, the thermo-chemical conversion remains

dominant because the technologies applied to the conversion of biomass into energy and fuels

are characterized by a highly efficient and sustainable operation with low environmental impact

[62]. Biomass can be thermally treated by gasification or pyrolysis. With these processes,

complex macromolecules found in biomass are broken into smaller molecules in gaseous or

liquid state (with remaining solid products such as ash and carbon), which can subsequently be

used as building blocks for further chemical synthesis [64].

21

Figure 1.6. Chemical transformation of biomass

Source: Adapted from R. Garcia, C. Pizarro, A.G. Lavin, J.L. Bueno, Bioresour. Technol. 103 (2012) 249–258.

Amongst all renewable sources, biomass is the only resource that can be directly converted into

high value end products using thermochemical conversion technology [65]. These technologies

rely on lignocellulosic biomass feedstocks (agricultural residue, forest residue) that do not

compete with food sources to form various fuels and chemicals [66]. The thermochemical

conversion of biomass to produce useful end products from initial feedstock can be achieved by

six different conversion techniques: pyrolysis, gasification, combustion, co-firing, liquefaction

and carbonization [67]. Biomass pyrolysis, having a long history of usage, has emerged as a

frontier research domain. It is generally defined as the thermal decomposition of the biomass

organic matrix in non-oxidizing atmospheres resulting in liquid bio-oil, solid biochar, and non-

condensable gas products [68]. Depending on the heating rate and solid residence time, biomass

Biomass

Thermochemical degradation

CombustionElectricity, Heat,

Steam

Distillation Coal, Gas, Tar

PyrolysisGas, Liquids,

Coal

Gasification Syngas

Hydro-gasification

Liquid Fuel

Chemical treatments

Acids, Alkali, Oxidants,

Organic Solvents

Alcohols, Furfural, Xylose,

Glucose

22

pyrolysis can be divided into three main types, including slow pyrolysis, fast pyrolysis, and flash

pyrolysis [69].

Bio-oil is a product of fast pyrolysis process, which is a dark brown, free flowing organic liquid

mixture. Bio-oil generally comprises a great amount of water, and hundreds of organic

compounds, such as acids, alcohols, ketones, aldehydes, phenols, ethers, esters, sugars, furans,

alkenes, nitrogen compounds and miscellaneous oxygenates, as well as solid particles [70]. The

abundant output of reviews and research papers on bio-oils in the last years is a glaring

indication of the vigorous growth of this area. Again, a search of the Web of Science database

using the words “pyrolysis bio-oil” identified a total of more than 3000 scientific articles

currently available on the subject that were published in the last decade (Thomson Reuters

2016). Most of these investigations are concerned with the optimization of the pyrolysis

parameters [68], as well as the use of bio-oil as bio-fuels [71,72], as an alternative to heat and

power generation [73], hydrogen production [74], and valued-added chemicals [75]. Due to the

complexity of bio-oil and the multitude of its components (which in turn depend on the source of

biomass, type of pyrolysis, temperature, residence time, etc.), analysis of bio-oil content often

employs instrumental techniques such as gas chromatography (GC), gas chromatography-mass

spectrometry (GC-MS), nuclear magnetic resonance (NMR), etc. The following Table (1.7)

depicts a model chemical group classification for bio-oil derived from wheat-wood sawdust [76].

23

Table 1.7. Chemical classification of bio-oil components

Chemical Classes Amount present in bio-oil (%)

Hydrocarbons 2.6

Low molecular weight fatty acids/esters 13.7

Hexadecanoic acid 23.6

Furanoids 3.2

Pyranoids 1.2

Benzenoid hydrocarbons 2.6

Oxygenated benzenoids 14.6

Low mol. wt. alcohols / aldehydes / ketones 1.7

High molecular weight alcohols 19.5

High molecular weight waxy compounds 2.1

Source: K. Jacobson, K.C. Maheria, A. Kumar Dalai, Renew. Sustain. Energy Rev. 23 (2013) 91–106.

All these components are derived primarily from depolymerization and fragmentation reactions

of the three building blocks of lignocellulosic biomass: cellulose, hemicellulose, and lignin. The

phenols, guaiacols, and syringols are formed from the lignin fraction. Miscellaneous oxygenates,

sugars, and furans are produced from cellulose and hemicellulose. The esters, acids, alcohols,

ketones, and aldehydes probably form as bi-products of the decomposition of oxygenated

compounds, furans, and sugars [19]. As seen, these pyrolysis technologies based on biomass

refinery appear as a good alternative to stand as a sustainable way for chemicals biosourcing.

However, in spite of the potential of the wood chemicals and forest derivatives, the biomass

refinery still facing several challenges that have limited its spread and extensive exploitation.

These factors include the feedstock diversity, biomass collection and transportation logistics,

seasonal variation, lad usage, compatibility with refinery infrastructure and facilities, market and

economic viability, sustainability and consistent R&D investments [77].

24

Concerning the last factor, government, academia, and industry have made significant

contributions in developing feedstocks and technologies to impulse biorefinery. Nevertheless,

many of these technologies remain in early stages of development. This stands as the key reason

why only very few studies considering the use of biorefinery products in the polymer synthesis

have been reported. On-going and consistent support, achieved through research and scientific

understanding, is essential to promote the development of this field and to make biorefinery a

profitable industry. In traditional bio-refinery, ethanol has provided opportunities to produce

hydrocarbon fuels and varieties of chemicals besides its application as fuel or fuels additive.

Ethanol can be converted into diethyl ether, ethylene, higher hydrocarbons or aromatics over

zeolites catalysts [78]. Likewise, efforts have been also focus on bio-butanols as a source of

chemicals and chemicals. At present, butanol has a wide range of market potentials as precursor

of solvents like butyl acetate, dibutyl acetate, butyl glycol ethers and butyl propionate. The

isobutanol has also potential as feedstock for synthetic rubbers, plastics and polyesters; such as

butyl acrylate, butyl methacrylate and butyl vinyl ether [78].

In the case of lignin, only small amounts of this material are presently used for commercial

purposes. Almost all lignin that is currently commercially used in non-energy applications is

ligno-sulfonate produced from spent sulfite pulping liqueurs in cement applications [79]. Issues

related to low reactivity associated to steric hindrance have surrounded the lignin for a long time;

however efforts to use lignin for the production of enhanced composites, carbon fibers, and

nanomaterials are found in literature [80]. Regarding the use of bio-oil as a source of chemicals

for macromolecular synthesis, very few studies have been published so far. Bio-oil chemistry

still in an early stage of development, and considerable work needs to be performed in order to

25

make it a profitable and fruitful source of chemicals. As it will be detailed later in Chapter V, this

fact opens up a huge opportunity to perform scientific research on this field; and the crude bio-oil

could serve as raw materials for the obtention and production of specialty chemicals and resins,

such as epoxy, phenol-formaldehyde, polyurethane, acrylic, etc.

Presently, the use of bio-oil in the polymer science has been addressed to partially replace

commercial resins. Wei et al. (2014) studied the use of bio-oil derived from switchgrass to cure a

commercial epoxy EPON 828 resin. Liquefaction time and temperature, as well as the optimal

epoxy / bio-oil ratio were studied [81]. On the same verge, Celikbag et al (2014) employed bio-

oil derived from loblolly pine to also crosslink EPON 828 [82]. The effect of the liquefaction

temperature and time on the hydroxyl number, as well as its impact of the thermo-mechanical

properties of the obtained materials were assessed.

Recently, vanillin and its derivatives have attracted attention as renewable building blocks for

high performance polymers, mainly because of its aromatic nature. Vanillin, and similar

aromatic compounds, has been identified in pyrolysis bio-oils [83]. Zhang et al. copolymerized

bio-monomer-methacrylated vanillin with acrylated soybean oil at different ratios [84]. The

materials were studied by dynamic-mechanical analysis, thermo-gravimetric and tensile tests.

26

1.6. Research objective

According to what has been reviewed, it is clear that the development of green polymers is

growing, and that it is supported not only by the industrial sectors, but also by governmental

entities and agencies. In order to approach the exploitation of natural resources as a source of

raw materials for the polymer industry, four different strategies can be defined [85], namely:

(1) The preparation of monomers which are already widely used, as obtained from fossil

resources.

(2) The synthesis of polymers which are intended to simulate the performance of existing

counterparts from non-renewable sources.

(3) Original materials with novel properties and applications.

(4) The chemical or physical modification of natural polymers with the intent of improving or

radically modifying their pristine properties.

The second strategy constitutes the corner stone of this research, and can be used meaningfully in

the construction and understanding of the work that is presented in the following sections; which

deal with the synthesis of novel polymers, intended to simulate the performance of existing

counterparts from non-renewable sources. Therefore, the objective of this project is the use of

monomers or macromonomers derived from the biomass, which bear reactive sites that can be

exploited, as such, or after appropriate modifications, to generate novel macro-molecular

architectures. Many studies have revolved around the development of biomass derived materials;

however, the obtention of polymers with good thermo-mechanical properties remains as a major

challenge, as well as the development of materials with higher contributions of biomass-derived

compounds per weight basis. This research stands as an effort to contribute to overcome these

27

difficulties, by producing biomass-derived thermosetting polymers with properties comparable to

their commercial counterparts. This document is divided into five sections. The first one explains

the overall organization of the work. The rest of them present an introduction, which main

purpose is to provide an insight about the major theoretical and technical aspects of the contents

and experimental results that are subsequently detailed.

The second section discusses the cationic copolymerization of tung oil, limonene and myrcene as

comonomers, initiated by boron trifluoride. Dynamic mechanical analysis (DMA) revealed that

all copolymers behave as thermosets. FTIR spectra for both copolymers, after extraction with

dichloromethane, suggested that the major component of the insoluble fraction was reacted tung

oil (a cross-linked triglyceride network). Likewise, unreacted tung oil was found to be the main

component of the soluble phase. Also, all the copolymers showed only one tan δ peak, indicating

no phase separation. Glass transition temperature (Tg) increased with the myrcene content and

decreased almost linearly as the limonene content increased. Furthermore, the Fox and Loshaek

model showed a relatively good prediction of the Tg values of the polymers. The Young`s

modulus ranged from 33.8 to 4.7 MPa for all tested thermosets.

The third one focuses on the chemical modification of two different functional groups of biomass

derived materials; the carbon-carbon double bonds of triglycerides, and the hydroxyl groups of

phenolic compounds. These chemical modifications were addressed to introduce more reactive

functional groups able to polymerize and to produce materials with a better thermo-mechanical

performance. Efforts are centered on the epoxy functional groups, and the feasibility of using

step-growth or chain-growth polymerizations for their conversion into macromolecular materials

28

is also assessed. Linseed oil was epoxidized using a peracetic acid generated in situ from reaction

between acetic acid and hydrogen peroxide, meanwhile α-resorcylic acid was reacted with

epichlorohydrin in alkaline medium using benzyltriethylammonium chloride as phase transfer

catalyst. The epoxy content of the modified monomers was measured by means of a titration

using HBr in acetic acid solution, and the grafting of epoxy functions onto the monomer`s

structure was studied by FTIR. The epoxidized derivatives of α-resorcylic acid and linseed oil

were cured in epoxy polymer with DMPA. Thermo-mechanical characterization showed that the

obtained materials behave as thermoset amorphous polymers, exhibiting moduli values in the

order of Giga Pascals at room temperature and glass transition temperatures above 100 °C. SEM

images confirmed the glassy brittle nature of the materials and revealed possible thermodynamic

incompatibility of these two compounds. When compared to samples of commercial origin,

testing showed the mechanical properties of the triglycidylated α-RA based resins are similar to

those shown by EPON 828 epoxy resin.

The fourth focus on the synthesis of interpenetrating polymer networks (IPNs) using an epoxy

phase synthesized either from linseed oil or α-resorcylic acid, and an acrylate phase employing

acrylated soybean oil are the main focus of this study. Thermo-mechanical characterization

showed that the obtained materials behave as thermoset amorphous polymers, and that both the

modulus and glass transition are extremely dependent on the epoxy/acrylate weight ratio, as well

as on the chemical structure of the epoxy monomers. In the case of the systems epoxidized

linseed oil / acrylated soybean oil, the mechanical properties of the IPNs were not superior to

those of the constituent polymers as a result of the plasticizing effect of the triglyceride flexible

chains in the macromolecular assembly of the IPNs. However, IPNs based on epoxidized α-

29

resorcylic acid and acrylated soy bean oil provided considerably better mechanical properties

than the system epoxidized linseed oil and acrylated soy bean oil. For the α-resorcylic acid and

soy bean oil system, modulus values ranged from 0.7 to 3.3 GPa at 30 °C, and glass transition

temperatures ranging from around 58 °C to approx. 130 °C. Both parameters were affected by

the epoxy/acrylate ratio, showing an increase as the epoxy proportion was enlarged.

Interpenetrating networks containing equivalent weight proportions of the parent resins showed

the highest fracture toughness of the series, exhibiting a KIc value of around 2.1 MPa·m1/2.

The final section discusses the use of fast pyrolysis bio-oils as precursors of thermosetting epoxy

resins. In this context, with the objective to synthetize a high performance bio-based epoxy

polymer, fast pyrolysis bio-oil (containing lignin fragments) was employed as a source of

phenolic compounds in the production of a bio-based polymeric network. The bio-oil was

reacted with epichlorohydrin and an aqueous solution of sodium hydroxide in the presence of

benzyltriethylammonium chloride as a phase transfer catalyst. The amount of free phenolic

hydroxyl groups before and after modification was quantified through P31-NMR spectroscopy;

and the epoxy content of the bio-oil upon chemical functionalization was measured by means of

a titration using HBr in acetic acid solution. Grafting of epoxy functions onto the monomer`s

structure was studied by FTIR. Likewise, α-resorcylic acid was also modified with reactive

epoxy moieties and used as a model to study the mechanical behavior of phenolic-based epoxy

polymers. The epoxidized derivatives of the bio-oil were cured in epoxy polymer with DMPA.

Thermo-mechanical characterization showed that the obtained materials behave as thermoset

amorphous polymers, exhibiting moduli values in the order of Giga Pascals at room temperature

and glass transition temperatures above 100 °C.

30

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41

CHAPTER II

Renewable Thermoset Copolymers from Tung Oil and Natural Terpenes

1. Introduction

The search for sustainable chemistry has led to the development of new synthetic routes to

produce polymers from renewable resources. Lately, particular attention has been paid to

biomass as a source of a wide range of biomaterials. Amongst many biomass derived materials,

it is important to highlight the potential use of vegetable oils. The employment of these materials

as platform chemical for polymer synthesis has several advantages, which include availability,

inherent biodegradability, high purity and low toxicity [1]. Vegetable oils or triglycerides are

fatty acid esters of the triol glycerol. Basically, a fatty acid consists of a hydrophobic

hydrocarbon chain with a hydrophilic group at one end. The degree of unsaturation varies

according to the composition of fatty acids presenting double bonds. This value, one of the most

important parameters in the chemistry of fatty acids and triglycerides, can be calculated through

the iodine value, a structural index which has been related to various physical and chemical

properties and has served as a quality control method in hydrogenation reactions [2].

Likewise, besides the carbon-carbon double bonds, some plant oils exhibit fatty acids containing

other functional groups such as hydroxyls, cyclic groups, epoxy groups, etc. which are amenable

for developing chemical reactions and structural modifications. According to the literature, there

are three main routes for the synthesis of polymers from plant oils. The first approach

42

corresponds to the direct polymerization through the C=C bonds [3,4]. Their presence allows the

polymerization of the oils into solid polymeric materials either through free radical or cationic

mechanisms [5–7]. The second route consists of the functionalization of the double bonds to

insert more reactive groups, usually epoxy, amine or hydroxyl, etc. [8,9]. They can be considered

as an intermediate product for subsequent reactions to yield bio-based polymers such as

polyurethane, polyester, etc. The third approach focuses on the chemical transformation of

vegetable oils to produce platform chemical which can be used to produce monomers for the

polymer synthesis [10].

Regarding the direct polymerization of the C=C, the free radical polymerization has received

little attention due to the presence of many side reactions. On the other hand, the cationic

polymerization has been studied by several researchers with successful results [11,12]. The

cationic copolymerization reaction usually requires an initiator. Several Lewis acids, such as

AlCl3, TiCl4, SnCl4 and BF3-OEt2 (BFE) amongst others, have been used as initiators to

copolymerize vinyl monomers cationically under relatively mild reaction conditions. As reported

by some researchers [13,14], BFE has proved to be a very efficient initiator in cationic

polymerization of vegetable oils. Usually, a trace of water or a diol, acting as a co-initiator is

needed to protonate the monomer. Triglycerides in general are cationically polymerizable

monomers. The accepted mechanism for such polymerizations is similar to that proposed for

cationic polymerization of vinyl monomers [15]. Each unsaturated fatty acid chain can

participate in the reaction, and the presence of several unsaturations leads to an extensive

crosslinking. Tung oil, containing a conjugated triene, is very reactive and highly susceptible to

polymerization via a cationic mechanism, because the intermediate carbocation can be stabilized

43

by resonance [1]. Likewise, vegetable oils can be polymerized with reactive monomers in order

to vary the properties of the oil-based matrix [16]. Due to the presence of multiple functional

sites per triglyceride molecule, which allow them to crosslink, oils can be considered as

analogous to vinyl esters and unsaturated polyesters [17].

Terpenes, natural monomers having carbon skeletons of isoprene units, are found in many

essential oils and represent a versatile chemical feedstock [18]. Even though the use of terpenes

in chemical synthesis is vast and well supported by a large number of publications, their

applications in polymer science are scarce and far between, usually limited to attempts to

polymerize bicylic monoterpenes, such as α and β pinene by Lewis acid and Ziegler-Natta

catalysis [19,20]. As stated by Norström [21], terpenes do not tend to polymerize, and their lack

of reactivity can be attributed to both steric hindrance and low stabilization energy between the

monomer and the chemical species in the transition state. However, since terpenes exhibit

carbon-carbon double bonds amenable to reactions, several works have been published

concerning the polymerization of terpenes with vinyl monomers in order to develop

biodegradable polymeric materials [22–24].

Envisioning the oil molecules as chains bearing multiple vinyl monomers, the main purpose of

this part of the project is to cover the major aspects related to the synthesis, characterization as

well as properties of new macromolecular bio-based thermoset polymers, produced through the

cationic copolymerization of tung (a natural oil) with three different terpenes; such as limonene,

myrcene and linalool as reactive comonomers. Also, in order to compare the properties of the

resulting materials, tung oil was copolymerized with styrene, a widely studied synthetic

44

monomer. The chemical, physical and mechanical properties of these new polymeric materials

were investigated as a function of the proportion and type of reactive monomer included in the

mixture. No other study has reported the copolymerization of plant oils and terpenes. The latter

can be considered as structural analogs to commercial vinyl monomers such as styrene, divinyl

benzene, etc.

2. Methods

2.1 Materials

Tung oil (TO), limonene (Lim), myrcene (Myr) and linalool (Lin) were purchased from VWR

International (US) and used with no further purification. Styrene (ST) was purchased from

Aldrich Chemical Company (US) and used as received. Tetrahydrofurane (THF) was used as an

initiator modifier and was supplied by VWR International (US). Boron Trifluoride Etherate

(BFE) was employed as an initiator and 1,4-butanediol was used as co-initiator; both were

acquired from VWR International (US).

2.2 Polymer Preparation Via Cationic Polymerization

The polymeric materials were prepared by cationic polymerization initiated by modified BFE.

The desired amount of the comonomer was measured out and added to the tung oil. The mixture

was vigorously stirred followed by the addition, drop by drop, of the appropriate amount of the

modified initiator. The modified catalyst was prepared by dissolving the boron trifluoride diethyl

etherate and 1,4-butanediol in the desired amount of THF. The reaction mixture was placed into

an aluminum mold and heated for a given time, usually 12 hrs. at room temperature, followed by

12 hrs. at 60 °C and finally for 12 hrs. at 110 °C.

45

2.3 Characterization

Soxhlet extraction was employed to characterize the structures of the bulk copolymers [25]. An

appropriate amount of the bulk polymer was extracted for 24 h with 200 mL of refluxing

dichloromethane with a Soxhlet extractor. Upon extraction, the resulting solution was

concentrated by rotary evaporation and subsequent vacuum drying. The soluble substances were

isolated for further characterization. The insoluble solid was dried in a vacuum oven for several

hours before it was weighed. FTIR spectra of both the soluble and insoluble fraction extracted by

Soxhlet were recorded. The densities of the copolymers were determined by pycnometry [26].

Mechanical properties of the resulting polymers were obtained by dynamic mechanical analysis

(DMA). To be able to explain the effects of the co-monomers on the mechanical properties, it is

useful to calculate the crosslinking density (n, mol/m³). The crosslinking density can be

estimated from the experimental data using the rubber elasticity theory. Thermosets behave as

rubbers above Tg. At small deformations, rubber elasticity predicts that the modulus storage (E’),

of an ideal elastomer with a network structure is proportional to the crosslinking density

according to Equation 2.1 [27]:

�� = 3��� = 3��/�� (2.1)

where E` is the rubbery storage modulus, R is the gas constant (8.314 J/mol K), T is the absolute

temperature (K), ρ is the density of the sample (g/m³), and Mc is the molecular weight between

crosslinks (g/mol). The temperature and rubbery modulus were determined for the calculation of

the equation at Tg + 30 °C. The temperature at which the peak of the tan delta presents a

maximum was considered the glass transition temperature of the material. In order to predict the

Tg based on the crosslinking density, the Fox and Loshaek model (which assumes specific

46

volume of the polymer at the glass transition as a linear function of Tg) was employed to study

the tung oil based copolymers. The Fox and Loshaek expression is shown in Equation 2.2 [17]:

� = � � + �/�� (2.2)

Where, K is a material constant, Tgu corresponds to the Tg predicted by the Fox equation, and Mc

corresponds to the molecular weight between crosslinking. The parameter K = 5.2 x 10³ K g/mol

was used to fit the model to the experimental results [17].

3. Results and Discussion

3.1. Cationic Polymerization of Vegetable Oils

Early in this research, several attempts to develop oil-based materials by cationic polymerization;

employing different types of vegetable oils, such as linseed oil, tung oil, soybean oil, corn oil,

etc. were carried out. Keeping in mind the preparation of completely bio-based polymers as the

major goal, the triglycerides were copolymerized with terpenes, natural monomers containing

alkenyl functional groups amenable to react.

Fruitful reactions leading to useful materials were achieved when tung oil, whose major

component is a fatty acid containing three conjugated unsaturations, was copolymerized with

limonene and myrcene as reactive comonomers (Figure 2.1).

47

Figure 2.1. Cationic polymerization of tung oil with limonene, myrcene and linalool

As pointed by Liu and Erham [11], the polymerization of 1,2 disubstituted ethylenes containing

small side groups easily occurs. Nevertheless, when bulky groups are attached to the double

bonds, the polymerization either does not occur or leads to low molecular weight products after

long reaction times. The steric hindrance plays a preponderant role during the propagation of the

growing chains. It introduces a high energy barrier between the monomer and the growing

species, allowing a very limited amount of polymer to be produced. Since fatty acids are 1,2

disubstituted ethylene monomers bearing bulky groups, their polymerization is considered

energetically unfavourable.

Unlike the other tested oils, tung oil was found to be highly reactive towards cationic and free

radical polymerizations. Its particular reactivity can be associated to its capacity to stabilize, by

resonance, the intermediate carbocation generated on the propagation step of the cationic

mechanism. In regards to the use of terpenes as comonomers, successful polymerizations were

R = α-Eleostearic Acid

=

Limonene Myrcene Linalool

Tung oil Crosslinked network

48

carried out when using limonene and myrcene. The polymerization of tung oil with linalool led

to highly porous materials. The gassing generated during curing could be attributed to side

reactions between the linalool and the boron trifluoride ethereate. All the copolymers appeared as

dark-brown rubbery materials at room temperature. For all the studied comonomers, suitable

materials for analysis were not obtained for terpene proportions above 60 wt%.

3.2. Characterization of the Polymer Networks

Once the reacting system has reached the gel point, the polymeric matrix is composed of two

fractions, the insoluble gel fraction and the soluble sol fraction. In order to characterize both

fractions, samples containing 80 wt% of tung oil (20 wt% comonomer) were Soxhlet extracted

using dichloromethane as a refluxing solvent. The study of the two phases, as well as the pure

tung oil (to track any possible change in its structure during curing) was developed through FTIR

spectroscopy.

The tung oil structure was characterized by the presence of a conjugated triene, which showed IR

absorption bands at 3010, 990 and 960 cm-1. The ester carbonyl exhibited two strong peaks at

1750 and 1150 cm-1. Methylene and methyl groups showed absorption bands at 2925 and 2854

cm-1, as well as at 1465 and 1370 cm-1. An inspection of the FTIR spectrum of the insoluble

fraction revealed that the major component in this phase was reacted oil (a cross-linked

triglyceride network). The spectrum presented similar absorption bands to those exhibited by the

tung oil. The most remarkable difference between them was the disappearance of the weak peak

at 3010 cm-1 (attributed to the presence of unsaturations in the tung oil structure). The lack of this

absorption peak in the FTIR spectrum of the insoluble fraction could be related to the fact that

49

the majority of double bonds had been consumed during the polymerization, either to react with

other fatty acid chains or with the comonomer, leading to the disruption of the conjugated triene,

responsible for the high reactivity of this oil. Upon extraction, for copolymers 80 wt% tung oil-

20 wt% comonomer, around 81-84 wt% of insoluble materials were retained. For all the

remaining copolymers, the retained insoluble materials ranged from 76 to 85 wt% after Soxhlet

extraction. FTIR analysis of the soluble fraction of the copolymer 80 wt% tung oil-20 wt%

comonomer suggested that the main component presented in this phase was unreacted oil.

3.2.1 Thermo-Mechanical Behavior

Dynamic mechanical analysis (DMA) measurements are intensively used to investigate the

amorphous phase transitions of polymers as they are stressed under periodic deformation. DMA

can be used to obtain the storage modulus and the loss modulus as a function of temperature at a

given frequency. Figure 2.2.a shows the temperature dependence of the storage modulus for the

copolymers tung oil-limonene. Results revealed that the materials behaved as thermosets. As

seen in the curves, the storage modulus initially remained approximately constant. As the

temperature increased, the storage modulus exhibited a drop over a wide temperature range

(glass transition), followed by a modulus plateau at high temperatures, where the material

behaved like a rubber. This region, called the rubbery plateau, evidences the presence of a stable

cross-linked network. The modulus drop is associated with the beginning of segmental mobility

in the cross-linked polymer network; whereas the constant modulus at temperatures above 30°C

is the result of the cross-linked structure of the copolymer [5]. The same behavior has been

reported by Li and Larock [7].

50

Figure 2.2. Temperature dependence of the (2.2.a) storage modulus (E’) for the copolymers tung

oil-limonene. (2.2.b) Loss factor (tan δ) as a function of the temperature for the tung oil-

limonene copolymers.

0% 10%20%

30% 40%

50%

60%

1 E+05

1 E+06

1 E+07

1 E+08

1 E+09

1 E+10

-50-40-30-20-10 0 10 20 30 40 50 60 70 80 90100

log

E`

(Pa

)

Temperature (°C)

0%

10%

20%

30%

40%

50% 60%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-50 -40 -30 -20 -10 0 10 20 30 40 50 60

Tan

δ

Temperature (°C)

2.2.b

2.2.a

51

The curves of the loss factor (tan δ) as a function of the temperature for the tung oil-limonene

copolymers are shown in Figure 2.2.b. The peak in tan δ corresponded to the main mechanical

relaxation of the matrix, which can be used as a criterion to determine the Tg of the copolymer.

As can be seen, the transition between the glassy to the rubbery state was wide and began below

room temperature for all the tested materials. Likewise, all the copolymers showed only one tan

δ peak, indicating that there was no phase separation. The results showed that the peaks shifted

to lower temperatures (indicating a decrease in the glass transition temperature), increased in

height and become narrower when increasing the co-monomer weight proportion. The peak

width at half height is a criterion used to indicate the homogeneity of the amorphous phase. A

higher value implies higher inhomogeneity of the amorphous phase. In this particular case, the

narrowing and increase in height of the loss factor peaks could be associated to less cross-linked

and heterogeneous networks with narrower molecular weight distribution of the chains between

crosslinking points [17].

Table 2.1 summarizes the properties of the tung oil-limonene copolymers. As shown, the storage

modulus, the crosslinking density and the Young`s modulus decreased when the tung oil

concentration increased. On the other hand, Tg decreased almost linearly with the increase in the

limonene content. Pycnometry showed variations in the density of the copolymers for

percentages of limonene higher than 30 wt%.

52

Table 2.1. Properties of the tung oil-limonene copolymers

Limonene

Proportion

(wt %)

Storage

Modulus

(MPa)

Tg

(ᵒC)

Crosslinking

Density, n

(mol/m³)

Density,

(g/cm³)

Young`s

Modulus

(MPa)

0 32.5 18.28 4118 1.0036 ± 0.0027 33.80 ± 2.92

10 17.8 13.89 2287 1.0097 ± 0.0020 14.95 ± 1.90

20 11.4 10.53 1480 1.0068 ± 0.0132 10.40 ± 0.26

30 4.5 5.58 594 1.0080 ± 0.0252 6.34 ± 1.62

40 2.7 4.33 357 0.9839 ± 0.0036 3.38 ± 0.31

50 0.8 0.17 112 0.9764 ± 0.0151 3.26 ± 0.07

60 0.5 -3.83 65 0.9414 ± 0.0081 2.22 ± 0.12

Fatty acid chains can contribute to two different effects, either plasticizing the network because

of the more flexible structure or increasing the rigidity due to the large number of unsaturations

per molecule [5]. Both factors can contribute to the properties of the copolymers. In the case of

tung oil-limonene materials, a decrease in both the number of crosslinking points and the glass

transition temperature of the networks was observed. As demonstrated by Norstrom [21],

limonene does not tend to homopolymerize (under the experimental conditions reported in

Norstrom`s work, polymerizing limonene units through cationic polymerization mainly yielded

to oligomers that consisted of trimers and tetramers). However, even though it is been clearly

stated that formation of large molecules (homopolymerization) out of this terpene not likely to

occur, several works have proved that limonene is able to react with vinyl monomers leading to

copolymers in which terpene units have been incorporated into the vinyl resin. Due to the

inability of limonene to form long chains (even at high weight proportions), here it is suggested

53

that the decrease in the number of crosslinking points arises as a result of limonene units that

have been incorporated into the fatty acid chains, lowering their ability to form an oil-based

cross-linked network. The decrease in the glass transition of the networks at high limonene

contents, in which crosslinking is very low, is driven by the glass transition of the limonene

oligomers (small linear segments) formed during heating of the polymeric matrices.

DMA of the tung oil-myrcene copolymers revealed a similar behavior to that shown by the tung

oil-limonene copolymers. When the temperature increased, the storage modulus exhibited a drop

over a wide temperature range, followed by a modulus plateau at high temperatures, evidencing

the presence of a stable cross-linked network. Also, the modulus decreased as the myrcene

proportion in the mixtures increased. The transition between the glassy to the rubbery state for

these copolymers was wide and began below room temperature. Likewise, for all the

copolymers, no phase separation was observed. The results showed that the tan δ peak increased

in height and become narrower as the myrcene content in the matrix increased. As in the case of

tung oil-limonene copolymers, this phenomenon could be attributed to the presence of less

heterogeneous networks. Unlike the tung oil-limonene materials, the maximum in the loss factor

shifted to higher temperatures when increasing the myrcene proportion above 10 wt%, indicating

that the glass transition temperature for these copolymers increased as the comonomer proportion

increased. Table 2.2 summarizes some of the properties of the tung oil-myrcene copolymers. As

shown, the glass transition temperature increased with the content of myrcene for percentages

higher than 10 wt%. Likewise, the increase in the comonomer proportion decreased the amount

of crosslinking points in the copolymers. The storage modulus, along with the Young`s modulus,

54

decreased with the content of myrcene. The pycnometry test did not show appreciable changes in

the density with the change in the monomer proportion.

Table 2.2. Properties of the tung oil-myrcene copolymers

Myrcene

Proportion

(wt %)

Storage

Modulus

(MPa)

Tg

(ᵒC)

Crosslinking

Density, n

(mol/m³)

Density,

(g/cm³)

Young`s

Modulus

(MPa)

0 32.5 18.28 4118 1.0136 ± 0.0057 33.80 ± 2.92

10 22.5 12.38 2905 1.0097 ± 0.0028 9.59 ± 1.72

20 14.6 15.31 1867 1.0098 ± 0.0082 5.36 ± 0.44

30 7.3 17.71 925 1.0102 ± 0.0152 3.22 ± 0.15

40 3.4 22.32 420 1.0134 ± 0.0360 1.31 ± 0.06

50 1.3 25.44 156 1.0092 ± 0.0101 0.59 ± 0.01

60 0.3 29.82 33 1.0109 ± 0.0181 0.21 ± 0.03

Myrcene is a natural conjugated diene that has been researched as a monomer for the preparation

of 100 wt% biosource elastomeric materials. Polymyrcene has been prepared by several

methods, including anionic polymerization with sec-BuLi and more recently, was

homopolymerized by a free radical process in aqueous media [28]. Myrcene yields polymers

with different structures, due to the three possible electrophilic additions to the conjugated diene

placed in the molecule; being the 1,4 addition the predominant one (77-85%) and 3,4 addition in

lesser degree (15-23%) [29]. Unlike other common 1,3-dienes (such as butadiene or isoprene),

myrcene provides polymers with structural units bearing an additional double bond in its alkyl

tail, which is very suitable for crosslinking.

55

When polymerizing myrcene and tung oil in the presence of BFE, the myrcene, contrarily to the

limonene, is not only able to react with the oil, but it is also capable of homopolymerizing and

creating linear myrcene-myrcene sequences. When increasing the amount of comonomer, the

diluting effect of the myrcene sequences immersed into the resin becomes predominant, and

hence, the crosslinking density decreases, consequently leading to a decrease in the storage

modulus. The increase in the glass transition of the networks when increasing the myrcene

content is attributed to the presence of these linear myrcene-myrcene sequences formed during

heating. As mentioned above, these linear polymyrcene sequences contain structural repeating

units bearing very short dangling chains having an extra double bond, which can be prompt to

react under the experimental conditions. At high myrcene concentrations, these additional

unsaturations can react and generate polymyrcene cross-linked networks. The bonding of these

polymyrcene linear segments (embedded in the matrix) through crosslinking, stiffs and strengths

the system and therefore; limits the segmental motion of the polymer chains, driving the glass

transition temperature to higher values whilst increasing the terpene content.

In order to compare the results obtained in this research with previous works dealing with

copolymerization of vegetable oils and synthetic vinyl monomers, tung oil was copolymerized

with styrene in the presence of boron trifluoride as an initiator, at room temperature. Table 2.3

summarizes the properties of the tung oil-styrene copolymers. The glass transition temperatures

(Tg) increased almost linearly with the content of styrene for percentages higher than 10 wt%.

The increase in tung oil concentration increased the amount of crosslinking points in the

copolymers. The storage modulus, along with the Young`s modulus, decreased with the content

56

of styrene. The pycnometry test also did not show any appreciable change in the density with the

variation in the monomer proportion.

Table 2.3. Properties of the tung oil-styrene copolymers

Styrene

Proportion

(wt %)

Storage

Modulus

(MPa)

Tg

(ᵒC)

Crosslinking

Density, n

(mol/m³)

Density,

(g/cm³)

Young`s

Modulus

(MPa)

0 32.5 18.28 4117 1.0136 ± 0.0057 33.80 ± 2.92

10 14.7 10.00 1913 1.0129 ± 0.0087 17.10 ± 2.07

20 10.7 12.04 1383 1.0158 ± 0.0033 10.17 ± 0.76

30 8.7 15.91 1109 1.0205 ± 0.0121 8.98 ± 0.35

40 6.8 20.15 858 1.0238 ± 0.0028 7.30 ± 0.67

50 4.7 27.32 584 1.0292 ± 0.0156 6.29 ± 0.23

60 2.9 30.47 354 0.9653 ± 0.0178 5.38 ± 0.39

Just like in the cases of tung oil-limonene and tung oil-myrcene, the modulus was found to be

strongly dependent on the comonomer proportion. It exhibited a drop over a wide temperature

range, followed by a modulus plateau at high temperatures, evidencing the presence of a stable

cross-linked network. All the copolymers showed only one tan δ peak denoting, like in the other

cases, the homogeneity of the materials. The height of the tan δ peak increased as the styrene

content in the matrix increased. The maximum in the loss factor shifted to higher temperatures

when increasing the comonomer weight proportion, denoting an increment in the glass transition

temperature. As myrcene, the styrene is not only able to react with the oil, but also, it is capable

of homopolymerizing and generating linear styrene-styrene sequences incorporated between the

57

oil matrixes. As increasing the amount of comonomer, the diluting effect of these linear

segments immersed into the resin became predominant; leading to a decrease in the crosslinking

density (consequently conducing to a reduction in the storage modulus). The increase in the Tg is

also attributed to the presence of these stiff and strong linear Polystyrene segments embedded in

the resins.

3.3. Theoretical Prediction of Tg Based on the Fox-Loshaek Model.

In order to predict the Tg of the copolymers, the Fox-Loshaek model (which assumes specific

volume of the polymer at the glass transition as a linear function of Tg) was considered. The

model can be considered as the sum of two contributions; the linear contribution Tgu, predicted

by the Fox equation, where the system is assumed to be formed by hypothetical linear molecules;

and the crosslinking contribution K / Mc, calculated through the Nielsen equation, which takes

into account the crosslinking of those hypothetical linear molecules [30].

To apply the model, the relation between the Tg and the crosslinking density was studied.

Through a simple mathematical approach, the evolution of the glass transition temperature of the

copolymers was found to be linearly dependent of the crosslinking density. Thus, employing

both the Fox and Nielsen equations, the two contributions for different compositions were

calculated. Figure 2.3.a shows the experimental values of the Tg measured by DMA as a

function of the limonene content, the linear contribution (Fox equation) and the Tg predicted by

the model. As seen, the model was able to predict the general trend in which the Tg decreased as

a function of the comonomer content. For the tung oil polymer (0% limonene) the model

overestimated the crosslinking density. For comonomer contents above 40 wt%, the model

58

underestimated the value of the crosslinking density. As previously mentioned, the decrease in

the Tg of the networks, at high limonene contents (in which crosslinking lowers), is dictated by

the low glass transition of the limonene oligomers. For proportions above 40 wt%, the effect of

these oligomers in the system becomes predominant; and the trend in the evolution of the glass

transition is driven towards low values.

260

265

270

275

280

285

290

295

300

305

0 10 20 30 40 50 60 70 80

Tg (

K)

Limonene proportion (wt.%)

Experimental Values

Linear (Fox Equation)

Tg copolymer

2.3.a

59

Figure 2.3. (2.3.a) Glass transition temperature of tung oil-limonene copolymers as a function of

the composition: (▲) values measured with DMA, (♦) linear copolymer contribution, (■) glass

transition temperature of the copolymer. (2.3.b) Glass transition temperature of tung oil-mycene

copolymers as a function of the composition: (▲) values measured with DMA, (♦) linear

copolymer contribution, (■) glass transition temperature of the copolymer.

The same approach was employed to study the evolution of the Tg as a function of comonomer

content for the tung oil-myrcene copolymers. As shown in Figure 2.3.b, the experimental data

closely fit the values predicted by the Fox equation. Even though the model predicted the general

trend for the variation in the glass transition temperature, is it important to denote that at high

concentrations of myrcene, it is capable to form linear molecules, and due to the presence of an

additional double bond, it is also able to generate cross-linked Polymyrcene molecules. Under

the experimental conditions, both structures are likely to be formed (the linear molecules and the

275

280

285

290

295

300

305

310

0 10 20 30 40 50 60 70 80

Tg

(K

)

Myrcene proportion (wt.%)

Experimental

Linear (Fox Equation)

Tg copolymer

2.3.b

60

cross-linked structure) and the generation of this reticulated Polymyrcene network is responsible

for the increase in the glass transition temperature observed in the experimental results. The

divergence between the experimental data and the values predicted by the model could be

associated to the fact that triglyceride-based polymers are very complex materials, containing

cross-linker molecules with a range of functionalities.

61

3.4. Conclusions

Thermosets were prepared through copolymerization of tung oil with limonene, myrcene and

styrene. The Tg was found to be a function of both the type and comonomer weight content. The

Young`s modulus and the experimental crosslinking densities decreased as comonomer weight

content increased. Based on the thermo-mechanical evaluation, these materials behave as

elastomers at room temperature. The Fox and Loshaek model showed a relatively good

prediction of the Tg values for tung oil-limonene copolymers. For myrcene, Tg values matched

those predicted by the Fox equation. Even though triglyceride-based polymers are complex

materials, some of the thermo-mechanical properties were found to be predictable by simple

mathematical approaches and models developed for polyvinyl resins.

62

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66

CHAPTER III

Triglycerides and Phenolic Compounds as Precursors of Bio-based Thermosetting Epoxy

Resins

1. Introduction

In recent years, there has been an increasing interest for the development and application of

biomass derived products to address issues related to the depletion of non-renewable resources.

Currently, the chemical industry is pursuing to substitute a growing part of fossil feedstocks with

renewable carbon. This trend is not only driven by environmental concerns, but it is also fueled

by the increase in the general demand for green products from costumers; as well as efforts from

government agencies providing funding to increase the present-day knowledge and technology to

process and transforming biomass into high-value chemicals [1]. In order to fulfill the needs of

the current industry, the polymer industry also demands a renewable chemical platform; and the

main challenge lies in developing materials with properties matching or even improving those of

resins in current use. In the particular case of thermosetting materials, which have played an

important role in the modern civilization, these properties include high modulus, strength,

durability, as well as the thermal and chemical resistances provided by the high crosslinking

density assembling their macromolecular structure [2].

In non-food applications, the most widely applied renewable resources include plant oils (only

castor and linseed oil are exclusively used of industrial applications) [3], polysaccharides

67

(mainly starch and cellulose), and proteins [2]. Ronda et al. [4] have considered three main

different approaches to convert oils and their derivatives into useful polymeric materials; the

direct polymerization of the double bonds, the chemical modification and posterior

polymerization, and the polymerization of monomers synthesized using oil-derived platform

chemicals. It is clear however, that this scheme can be extended not only to oils, but also to any

other type of biomass-derived compounds, leading to three different areas to generate

macromolecular materials, including direct use, chemical functionalization, and biorefinery.

As seen in Chapter II, direct polymerization of oils leads to obtain materials with marked

elastomeric nature, showing low Tg and modulus. Thus, chemical modification stands as a

common strategy for obtaining high performance polymeric materials. Carbon-carbon double

bonds can be converted into epoxides through a catalyzed oxidation using peracids, process that

is commonly known as the Prisleschajew reaction. The epoxidation takes place in a one-step,

concerted reaction that maintains the stereochemistry of any substituent on the double bond [5].

The epoxidized products could be diversified, because of the possibility of mono-, di-, or tri-

epoxides according to the origin of the oilseed [6]. Likewise, chemo-enzymatic epoxidation has

been study, mainly because it avoids the ring-opening of epoxide and the formation of

bioproducts, which are the most common drawbacks of the oxidation method [7].

Several studies have investigated the development of thermoset plastics and coatings from plant-

based oils. Zhu et al. [8] studied the development of inexpensive epoxy resins using soybean oil

derivatives cured with a commercial cycloaliphatic amine. Likewise, Shah and Ahmad

developed waterborne vegetable epoxy coatings, also curing the oil-based epoxy monomers with

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anhydrides [9]. Also, epoxidized soybean oil composites have been prepared by reinforcing the

oil matrix, cured with commercial amines, with flax fibers [10]. A complete comparison of

curing agents for epoxidized oils is given by Espinosa-Perez et al. [11]. It is important to

highlight that the above mentioned studies employed the step-growth process to formulate the

epoxy resin, in which a commercial crosslinking agents such as amines, anhydrides, etc. are

required to crosslink the epoxy monomers. This approach generates partially bio-based resins.

Scarce studies have dealt with chain-growth copolymerization, approach that requires a small

amount of an initiator to generate the polymer, leading to final materials with lesser contribution

of commercial compounds per weight basis. Kim and Sharma [12] reported the

homopolymerization of oil-based epoxy resins by using N-benzyl pyrazinium

hexafluorantimonate. It is also important to point out that the inherit long chain aliphatic

structure of oil-based thermoset monomers limits the polymer glass transition temperatures

compared to commercial, aromatic based monomers, such as DGEBA [13]. This outcome stands

up as a key finding, because opens the possibility for the study of the copolymerizing of oil-

based monomers with low molecular aromatic compounds; evaluating whether or not these

compounds can improve the mechanical properties of the oil-based resins. Indeed, the thermal

and mechanical performance properties of organic polymers are generally linked to the presence

of rigid aromatic structures [14].

Hence, biomass is also a potential source of aromatic compounds. Lignin is the second most

abundant naturally occurring macromolecule, and due to its phenolic nature, it has been

considered as a promising source of high value aromatic compounds. Some novel technologies

based on biomass refinery appear as a good alternative to produce high valued chemical from the

69

residues of pulping industries [15]. Likewise, cashew nut shell, an agricultural by-product

abundantly available in tropical countries, is one of the major resources of naturally occurring

phenols [2]. It contains four major aromatic components, namely cardanol, cardol, anacardic acid

and 2-methylcarbol. The viability of using these compounds as building blocks for epoxy resins

has been already a subject of scientific studies [16]. In a lesser degree, polyphenols, more

specifically from condensed tannins extracted from wastes produced by the wood and wine

industries can also be an alternative to produce epoxy resins [17].

In the previous chapter, the direct polymerization of the double bonds present in the fatty acid

chains of the triglycerides was studied, and the copolymerization with terpenes, cyclic

compounds structurally resembling commercial monomers such as styrene and divinyl benzene,

was also a subject of evaluation. The first part of this chapter focuses on the chemical

modification of the double bonds of plant oils. These chemical modifications were addressed to

introduce more reactive functional groups able to polymerize and to produce materials with a

better thermo-mechanical performance. Three chemical modifications (epoxy, hydroxyl and

acrylate) are preliminary evaluated. Efforts are centered on the epoxy functional groups, and the

feasibility of using step-growth or chain-growth polymerizations for their conversion into

macromolecular materials is also assessed.

The second part of this research covers the copolymerization of oil-based epoxy monomers with

an aromatic low molecular weight comonomer, chemically modified with oxirane groups, to

evaluate whether or not the increase in the density of aromatic structures in the crosslinked

network can improve the mechanical properties of the resulting materials. Thus, by employing

70

linseed oil and α-resorcylic acid as a model, functionalized with epoxy groups (via oxidation and

nucleophilic addition respectively); the main purpose of this chapter is to cover the major aspects

related to the chemical synthesis, physical-chemical characterization and study of thermo-

mechanical properties of a bio-based epoxy resins. α-resorcylic acid is a constituent of peanuts

(Arachis hypogea), chickpeas (Cicer arietinum), red sandalwood (Pterocarpus santalinus), and

hill raspberry (Rubus niveus) [18]. Likewise, α-resorcylic acid derivatives have been found as a

secondary metabolite in cultures of the fungus Sporotrichum laxum ATCC 15155 [19].

2. Experimental Section

2.1 Materials

Linseed oil (LO) was purchased from VWR International (USA) and used with no further

purification. Glacial acetic acid (99.5 %), hydrogen peroxide (30 %), sodium carbonate, toluene,

petroleum ether, ethyl acetate, iodine monochloride, Wijs’ solution, 33 % HBr in glacial acetic

acid, and Amberlite ion-exchange resin were all AR grade and also acquired from VWR

International (USA). α-Resorcylic acid (α-RA) ( (≥ 98%), epichlorohydrin (99.0 %), sodium

hydroxide (≥ 98%), and dichloromethane were purchased from VWR (US).

Benzyltriethylammonium chloride (≥ 98%), used as a phase transfer agent, was also acquired

from VWR (US). 4-dimethylaminopyridine (DMAP, 99.0 %), used as initiator for the chain

homopolymerization of epoxy monomers, and EPON 828 (with an epoxy equivalent weight =

185-192 eq/gr [20]) were purchased from VWR and used as received.

71

2.2 Synthesis procedures

2.2.1 Synthesis of epoxidized triglycerides.

Linseed oil (LO) was epoxidized using a peracetic acid generated in situ from reaction between

acetic acid and hydrogen peroxide, with ion-exchange resin catalyst, at 60 °C in toluene, during 8

h. [21,22] . A solution of linseed oil (20.1 g, 67.0 mmol) and acetic acid (1.0 mL, 26.2 mmol) in

toluene, using Amberite as an ion exchange catalyst, was stirred at 35 °C, and H2O2 30% w/v

was added dropwise. After H2O2 addition was complete, the temperature was raised to 60 °C and

the flask contents were stirred vigorously for 8 h. Once the reaction time was completed the

mixture was first dissolved in ethyl acetate. The Amberlite was filtered off, and the filtrate was

poured into a separation funnel, and washed with brine until the pH was neutral. The oil phase

was further dried above anhydrous sodium sulfate and then filtered. Finally, the solvent was

removed using a rotary evaporator.

2.2.2 Synthesis of polyol.

Polyol was synthesized from epoxidized linseed oil, with an oxirane oxygen content of 0.653

mol/100g, through a ring opening reaction with boiling methanol in the presence of

tetrafluoroboric acid as acid catalyst following the procedure described by Dai et al. [23]. The

molar ratio of epoxy groups to methanol was 1:11, and the concentration of the catalyst was 1

wt% of the total weight of oil and methanol. Methanol and catalyst were placed into the reactor

and heated to approx. 60 °C using an oil bath. Once the temperature was reached, the epoxidized

linseed oil was added dropwise. The reaction mixture was kept for 1 hour. After cooling to room

temperature, the acid was neutralized with sodium bicarbonate aqueous solution (approx. 30

wt%) and the remaining solvent was removed on a rotatory evaporator under low vacuum.

72

2.2.3 Functionalization of phenolic compounds with epoxy groups. Synthesis of

triglycidylated ether of α-resorcylic acid.

The glycidylation of bio-oil was conducted following the method and conditions established by

[24,25]. The described process leads to the synthesis of epoxyalkylaryl ethers by the reaction

between compounds containing acid hydroxyl groups and haloepoxyalkanes in the presence of a

strong alkali. A reactor was loaded with the desired amount of bio-oil (containing a certain

amount of moles of -OH / gram of oil) and epichlorohydrin (5 moles epichlorohydrin / mole -

OH). The mixture was heated at 100 °C and benzyltriethylammonium chloride (0.05 moles

BnEt3NCl / mole -OH) was added. After 1 h, the resulting suspension was cooled down to 30 °C,

and an aqueous solution of NaOH 20 wt. % (2 moles NaOH / mole –OH and also containing 0.05

moles of BnEt3NCl / mole –OH) was added dropwise. Once the addition of the alkaline aqueous

solution was completed, the mixture was vigorously stirred for 90 min at 30 °C, and then the

reaction was stopped. After that, the mixture was poured into an extraction funnel and the

organic layer was separated. Through numerous liquid-liquid extractions with deionized water,

the organic phase was abundantly washed until neutral pH was reached, and then vacuumed

concentrated. The crude resulting product was purified by silica gel chromatography using

petroleum ether/ethyl acetate solvent system.

2.2.4. Synthesis of bio-based epoxy resins.

In order to generate a polymeric material, epoxy monomers can undergo step-growth

polymerization, using a crosslinking agent (such as anhydrides, amines, etc.) and chain

homopolymerization in the presence of both, Lewis acids and bases, such as tertiary amines

(known as anionic homopolymerization). As a first approach, epoxidized linseed oil was cured

73

with a commercial hardener (SC-15 part B by Applied Poleramic Inc. composed of

cycloaliphatic amine and polyoxyl-alkylamine) in different weight ratios. The mixture was cured

for 6 hrs. at 60 °C, 6 hrs. at 80 °C, 2 hrs. at 120 °C and finally 1 hr. at 160 °C. In order to

minimize the use of commercial agents in the formulation, epoxidized linseed oil and epoxidized

α-resorcylic acid were mixed in different weight proportions and cured by means of a chain

Homopolymerization. 4-dimethylaminopyridine or DMPA (a Lewis base) was added to the

mixtures in a ratio of 0.08 mol/mol epoxy groups [26]. The mixtures were then poured in an

aluminum mold and cured at 120 °C for 12 hrs and post cured for 2 hrs at 160 °C.

2.2.5. Preparation of the linseed oil based polyurethane.

The PUR plastic sheets were prepared following the procedure described by Kong and Narine

[27] by reacting the resulting polyol with aromatic TDA. As a first approach, a 1.0/1.1 molar

ratios of the -OH group to the isocyanate (NCO) group (Mratio), was chosen for the formulation.

The desired OH/NCO molar ratio satisfies the Equation 3.1:

������ = � ������ �� �������

(� ������ !"#$ %� ������)�� '(�)�#$#! �

(3.1)

Here Wpolyol is the weight of the polyol, EWpolyol is the equivalent weight of polyol, WPU is the

total weight of polyurethane to produce, and EWisocyanate is the equivalent weight of the

isocyanate. The equivalent weights of polyol were determined using Equation 3.2:

�* +�,-�, = ./001%23 $�45 � (3.2)

The weight of the polyol and isocyanate were calculated using the above calculated equivalent

weight. Suitable amounts of polyol and MDI were weighed in a plastic container and stirred

slowly for 2 min. The mixture was cast directly into a metallic mold previously greased with

74

silicone release agent and placed in an oven at 80 °C for polymerization. The samples were

postcured at 120 °C for 12 h.

2.3. Characterization of Starting and Modified Materials

2.3.1 Fourier Transform Infrared Spectroscopy (FTIR)

With the aim to study the functional groups present in the product, FTIR was performed. Infrared

spectra of the starting materials and epoxidized products were measured by attenuated total

reflection (ATR) method using a Thermo Nicolet 6700 Fourier transform infrared spectrometer

connected to a PC with OMNIC software analysis. All spectra were recorded between 400 and

4000 cm-1 over 64 scans with a resolutions of 16 cm-1.

2.3.2 Epoxide equivalent weight determination.

Epoxide equivalent weight (EEW, g/eq.) of the epoxy products was determined by epoxy

titration according to Pan et al. [28]. Basically, a solution of 0.1 M HBr in glacial acetic acid was

used as a titrant, 1 wt. % crystal violet in glacial acetic acid was used as indicator, and toluene as

solvent for the sample. The EEW was calculated using Equation 3.3:

��* = 0111 6 �7 6 8 (3.3)

Where W is the sample weight (g), N is the molarity of HBr solution, and V is the volume of HBr

solution used for titration (mL).

For triglycerides, from the oxirane content values, the relative fractional conversion to oxirane

can be calculated by means of the following expression:

Relative conversion to oxirane = OOexp

OOth (3.4)

75

Where OOexp is the experimentally determined content of oxirane oxygen in 100 grs of oil, and

OOth is the theoretical maximum oxirane oxygen in 100 grs, which is given by:

99�ℎ = {(<= ̥)/2@)/[100 + DE8̥FGH @̥]} K @̥ K 100 (3.5)

Where A (126.9) and A0 (16.0) are the atomic weights of iodine and oxygen, respectively, and

IVo is the initial iodine value of the samples.

2.3.3 Iodine value determination

Iodine value (IV, g I2/g oil.) of linseed oil was determined by titration according to the Wijs

method [29]. Around 0.5 grs of the sample were placed into the flask. 10 mL of toluene was

added to the sample. After that, 15 mL of the Wijs iodine solution was added. Following a

similar procedure, a blank solution was prepared. The mixtures were stored in the dark for 30

minutes at room temperature and then 10 mL of 15 wt. % potassium iodide solution, as well as

50 mL of distilled water, were added to the mixtures. The iodine content was determined by

titration using 0.1 N of sodium thiosulfate solution, using starch aqueous solution as an indicator,

until the color of the solution disappeared. The iodine value was then calculated using Equation

3.6:

<= = (L%M)6 7N 0F./P� (3.6)

Where W is the sample weight (g), N is the normality of Na2S2O3 solution, B is the volume of

solution Na2S2O3 solution used for titration (mL) of the blank, and S is the volume of solution

Na2S2O3 solution used for titration (mL) of the samples.

76

2.3.4 Hydroxyl number determination.

The hydroxyl number was evaluated following the procedure described by the ASTM D4274-11

[30]. Around 0.5 grs of the sample were placed into a pressure bottle, and then 20.0 mL of

acetylation solution (acetic anhydride in pyridine) was added. The bottle was swirled until the

sample was completely dissolved. The containers were placed in a water bath at 98°C and kept

for two hours. The excess of reagent is hydrolyzed with water and the acetic acid is titrated with

standard sodium hydroxyl solution 0.5 N using phenolphthalein as indicator. The hydroxyl

content is calculated from the difference in titration of the blank and sample solutions. The

hydroxyl number, mg KOH/g, of sample was calculated as follows:

Q-R��K-, �STUV� = [(W%X)(Y)(./.0)]Z (3.7)

Where A = NaOH required for titration of the sample (mL), B = NaOH required for titration of

the blank (mL), N = normality of the NaOH, and W = sample used (g).

2.4 Characterization of the epoxy networks

2.4.1 Soxhlet extraction.

Soxhlet extraction was employed to characterize the structures of the bulk copolymers following

the ASTM D5369-93 [31]. An appropriate amount of the bulk polymer was extracted for 24 h

with 200 mL of refluxing dichloromethane with a Soxhlet extractor. Upon extraction, the

resulting solution was concentrated by rotary evaporation and subsequent vacuum drying. The

soluble substances were isolated for further characterization. The insoluble solid was dried in a

vacuum oven for several hours before it was weighed.

77

2.4.2 Scanning Electron Microscopy (SEM)

SEM study was performed with the purpose of obtaining a topological characterization of the

materials. The fractured surfaces of pre-chilled samples in liquid nitrogen were studied by

scanning electron microscope (SEM, Cambridge 360). Samples were sputtered with gold prior to

SEM observations.

2.4.3 Thermal Properties

The thermal properties of the IPNs, as well as the parent resins, were investigated by Differential

Scanning Calorimetry (DSC) on a TA DSC Q2000. Nitrogen was used as the purge gas and the

samples were placed in aluminum pans. For each material sample, the thermal properties were

recorded at 5 °C/min. in a heat/cool/heat cycle from 30 °C to 300 °C.

2.4.4 Thermo-mechanical properties of the networks.

Mechanical properties of the resulting polymers were obtained by dynamic mechanical analysis

(DMA), using a TA Instruments RSA3 DMA. To be able to explain the effects of the co-

monomers on the mechanical properties, it is useful to calculate the crosslinking density (n,

mol/m³). The crosslinking density can be estimated from the experimental data using the rubber

elasticity theory. Thermosets behave as rubbers above Tg. At small deformations, rubber

elasticity predicts that the modulus storage (E’), of an ideal elastomer with a network structure is

proportional to the crosslinking density according to Equation 3.8 [32]:

�� = 3��� = 3��/�� (3.8)

where E` is the rubbery storage modulus, R is the gas constant (8.314 J/mol K), T is the absolute

temperature (K), ρ is the density of the sample (g/m³), and Mc is the molecular weight between

78

crosslinks (g/mol). The temperature and rubbery modulus were determined for the calculation of

the equation at Tg + 30 °C. The temperature at which the peak of the tan delta presents a

maximum was considered as the glass transition temperature of the material. The Fox equation

was employed to fit the experimental Tg data reported in the results. It serves as a way to

describe Tg of the multicomponent networks as a function of relative epoxy monomer content.

The Fox expression is shown in Equation 3.9 [33]:

0

[\ = �0[\0 + �F

[\F (3.9)

In this expression, W1 and W2 represent weight fractions of components 1 and 2 (α-resorcylic and

linseed oil, respectively), and Tg1 and Tg2 are Tg values of polymer samples containing only

components 1 and 2.

3. Results and Discussion

3.1 Synthesis of epoxidized triglycerides

Previous investigations (Chapter II) revealed that at room temperature, all the polymers are in the

transition from the glassy state to the rubbery state, when using direct polymerization as an

approach to obtained triglycerides-based polymeric materials (without any chemical

modification of the starting materials). Since one of the main goals of this research is to develop

and synthetize polymeric materials from renewable resources with high performance and

outstanding properties (such as high modulus, high strength, high glass transition, chemical

resistance, etc.); the driving force behind this project is to explore and study the chemical

functionalization of plant oils. The modification of the triglyceride double bonds to introduce

readily polymerizable functional groups has been used as a common strategy for obtaining high

performance polymeric materials. The unsaturated fatty acid content of commercially available

79

vegetable oils varies depending on the fatty acid composition. It determines the degree of

unsaturation of the vegetable oil, and consequently, its reactivity. Also, the degree of

unsaturation defines the content of oxirane rings, when undertaking epoxidation reactions. Table

3.1 shows the degree of unsaturation, along with the maximum theoretical oxirane content after

epoxidation of some common oils [34].

Table 3.1. Properties of some common oils.

Oil Degree Unsaturation

Oxirane Content

(gr/100 gr sample)

Linseed 6.25 10.2

Soybean 4.61 7.95

Canola 3.91 6.59

Source: A. Zlatanic, C. Lava, W. Zhang, Z.S. Petrović, J. Polym. Sci. Part B Polym. Phys. 42 (2004) 809–81.

As seen in Table 3.1, linseed oil exhibits the highest degree of unsaturation, and thus, maximum

theoretical oxirane content. The linseed oil based epoxy resin (ELO) was synthesized by

epoxidation of the raw oil, using peracetic acid generated in situ (see Figure 3.1).

Figure 3.1. Epoxidation of linseed oil triglycerides

It was found that the optimum molar ratio of C=C/CH3COOH/H2O2 for obtaining the maximum

conversion of C=C into epoxy rings, minimizing side reactions, was 1/0.5/2 at 60 °C during 8

O

O

O

O

O

R O

R

CH3

O O O

O

O

O

O

O

R O

R

CH3

Linseed oil triglyceride

Epoxidized linseed oil triglyceride

80

hrs. The iodine value and the oxirane value are important characteristics in the epoxidation of

oils. The iodine value specifies the unsaturation degree of the oils, whereas the oxirane value

determines epoxy content of the modified oil. Once the reaction was completed and the product

isolated and purified, both parameters were quantified through titrations. According to the

experimental results, the conversion of the double bonds to epoxy groups during the epoxidation

reaction was found to be around 90%. From iodine values before and after epoxidation, it was

found that around 98% of the total double bonds were removed during reaction. This indicates

that approximately 8% of epoxy groups were lost due to side reactions. Titration with HBr in

acetic acid revealed that this material has an average molar epoxy content of (5.8 ± 0.7) x 10-3

moles per gram of resin. FTIR analysis supported the grafting process of epoxy functions onto

the fatty acids chains of the triglyceride (Figure 3.2). The FTIR spectrum of triglyceride is

characterized by the absorption band of -C=C- at 3100 cm-1 and the carbonyl group at 1750 cm-1.

After epoxidation, the peak at 3100 cm-1 is removed due to the transformation of the double

bonds into oxirane groups; which is also evident by the presence of the absorption peaks at

approximately 820 cm-1.

81

O

O

O

O

R

O

O

R

CH3

O O O

O

O

O

R

O

O

R

CH3

OH

CH3O

OH

O

CH3

CH3OH

Figure 3.2. FTIR spectrum of epoxidized linseed oil.

3.2 Synthesis of linseed oil based polyol

The linseed oil based polyol was synthesized by epoxidation of the raw oil, and its subsequent

ring opening reaction with refluxing methanol using tetrafluoroboric acid as acid catalyst (Figure

3.3).

Figure 3.3. Methanolysis of epoxidized linseed oil.

During the methanolysis reaction, the acid media promotes the oxirane ring opening in order to

generate a hydroxyl functional groups attached to the fatty acid chains. In the first part of the

O

O

O

O

O

R O

R

CH3

-C=C-

-C-H

-C=O

O

O

O

O

O

R O

R

CH3

O O O

-C-H

-C=O O

Linseed Oil

Epoxidized Linseed Oil

82

mechanism, the acid reacts with the epoxide to produce a protonated epoxide, a highly

electrophilic intermediate susceptible to the nucleophilic attack of the methanol molecules. Once

the reaction is completed and the product isolated and purified, the number of –OH groups

generated during reaction can be quantified through titrations. Table 3.2 summarizes the

properties of the obtained polyol.

Table 3.2. Properties of the resulting linseed oil based polyol.

Parameters

Hydroxyl Value HV

(mg KOH/g) (*)

Conversion

(%)

Theoretical

Molecular Weight

MW (g/mol)

Functionalitya

(*)

Values 230 (307) 75.0 % 1072 4.2 (5.8)

a. Functionality = [(MW x HV)/56110] * Parenthesis indicates theoretical values

Using the composition of the linseed oil, the theoretical molecular weight and the theoretical

hydroxyl number were calculated. Both parameters were based on the assumption that no dimers

or timers were formed, and that one hydroxyl group was formed per double bond. As seen, the

conversion from epoxy groups to hydroxyl groups reached approx. 75.0% of yield. Just like in

the case of the epoxidation reaction, there is side reactions involved in the ring opening reaction.

During the hydroxylation stage, the newly formed –OH groups, attached to the fatty acid chains,

might attack the protonated epoxy ring leading to the formation of dimers and trimmers, which

lowers the reaction yield. Another important parameter of a polyol, obtained from the hydroxyl

value, is the functionality; which allows calculating the correct stoichiometric ratios when

mixing polyols with multifunctional urethanes in order to produce polyurethanes. The

functionality of the resulting polyol is lower than the number of double bonds per molecule of

83

the starting oil, because a part of the functional groups was lost in the side reactions during the

preparation of the polyol [35].

Besides analytical quantification, the chemical transformation of carbon-carbon double bonds

into hydroxyl groups was also studied and traced through FTIR. As seen in Figure 3.4, the

absorption bands of epoxidized triglycerides appeared within 1720-1700 cm-1 for C=O, at 825

and 845 cm-1 for epoxy groups, and the absence of the C=C stretching band at approximately

3070 cm-1. The final polyol product showed a broad peak within 3500-3200 cm-1, indicating the

presence of –OH groups and the disappearance of the epoxy bands at 825 and 845 cm-1.

Figure 3.4. FTIR spectrum of hydroxylated linseed oil.

O

O

O

O

O

R O

R

CH3

O

O

O

O

O

R O

R

CH3

O O O

O

O

O

O

R

O

O

R

CH3

OH

CH3

O

OH

O

CH3

Linseed Oil

Epoxidized Linseed Oil

Hydroxylated Linseed Oil

84

3.3 Functionalization of phenolic compounds with epoxy groups.

3.3.1 Synthesis of triglycidylated ether of α-resorcylic acid

Regarding the coupling of epoxy groups to the α-resorcylic acid, it was found that the treatment

of this compound with epichlorohydrin in alkaline aqueous medium (see Figure 3.5), in the

presence of benzyltriethylammonium chloride as a phase transfer catalyst, allowed the total

glycidylation of this phenolic acid to generate the triglycidylated derivative of α-resorcylic acid

or TRA (also referred to as ERA, a short version of epoxidized α-resorcylic acid).

Figure 3.5. Glycidylation reaction of α-resorcylic acid with epichlorohydrin.

The reaction yield, after purification through silica gel chromatography, was around 80 %. The

remaining 20 % probably correspond to mono and di glycidilated products of α-resorcylic acid,

as well as, unreacted material. Because phenols are more acidic than alcohols, they can be

converted to phenolates through the use of a strong base. These intermediates are strong

nucleophiles and can react with primary alkyl halides to form ethers. According to the literature

[36], once the phenolate anions are formed, they can react with epichlorohydrin following two

competitive mechanisms: one-step nucleophilic substitution (mechanism SN2) with cleavage of

the C−Cl bond and a two-step mechanism based on ring opening of epichlorohydrin followed by

intramolecular cyclization (through SN1 mechanism), with the release of chloride, to reform the

epoxy ring. Titration with HBr showed that resulting mixture after modification has an average

OHOH

O OH

O O

O O

O

O

O

O

Cl

α- Resorcylic Acid Triglycidylated ether of α-Resorcylic Acid (TRA)

85

molar epoxy content of (9.0 ± 0.5) x 10-3 moles per gram of resin, which corresponds to an

epoxy equivalent weight = 111 gr/eq. The presence of epoxy groups was confirmed by FTIR

(Figure 3.6).

Figure 3.6. FTIR spectra of α-resorcylic acid and epoxidized α-resorcylic acid

The α-resorcylic acid structure was characterized by the presence of a strong IR absorption band

at 3200 cm-1. The carbonyl exhibited a strong peak at 1750 cm-1. The spectrum of the epoxidized

α-resorcylic acid presented similar absorption bands to those exhibited by its non-modified

counterpart. The most remarkable difference between them was the disappearance of the weak

peak at 3200 cm-1 (attributed to the presence of hydroxyl groups). The lack of this absorption

peak in the FTIR spectrum of the epoxidized α-resorcylic acid is related to the removal of the

-OH functional groups due to the glycidylation reaction. It is also evident the presence of

absorption bands at 1227.5 cm-1 and 1050 cm-1 due to the presence of -C-O-C- groups, formed

O OH

OHOH

O O

OO

O

OO

-OH

-C-H

-C=O

-C=O

O

-C-O-C

α- Resorcylic Acid

Triglycidylated ether of α- Resorcylic Acid

86

during the reaction, as well as the characteristic bands for oxirane rings at 847.5 cm-1 and 910

cm-1.

3.3.2. Synthesis of diglycidylated ether of α-resorcinol

With the aim of correlating the mechanical properties of the resulting resins with the number of

functionalities of the epoxy monomers, resorcinol (containing two phenolic –OH groups) was

also modified with epichlorohydrin (see Figure 3.7). The reaction yield, after purification

through silica gel chromatography, was around 85%. The remaining 15% probably corresponded

to monoglycidylated products and unreacted resorcinol. Titration with HBr showed that the

resulting mixture after chemical modification has an average molar epoxy content of (8.7 ± 0.5)

x 10-3 moles per gram of resin, resulting in an epoxy equivalent weight = 115 g/eq. The presence

of epoxy groups was also confirmed by FTIR (Figure 3.8). The lack of the absorption peak at

around 3200 cm-1 evidenced the transformation of phenolic hydroxyl groups into glycidyl

moieties. Bands for oxirane rings at 847.5 cm-1 and 910 cm-1 confirmed the presence of epoxy

groups.

Figure 3.7. Glycidylation reaction of resocinol with epichlorohydrin.

O

Cl

Resorcinol Diglycidylated ether of resorcinol

OHOH

OO

O O

87

Figure 3.8. FTIR spectra of resorcinol and epoxidized resorcinol

3.4. Thermo-mechanical characterization of bio-based epoxy thermosetting resin

3.4.1. Polymerization of epoxidized linseed oil through step-growth process

The first approach to study viability of chemical modification of the vegetable oils, as well as its

effect on the resulting thermos-mechanical properties, included the introduction of epoxy groups

on the fatty acid chains of linseed oil. The storage modulus (E’) and the glass transition (tan δ)

of thermally cured epoxidized thermosets were used as a guide to evaluate the thermo-

mechanical performance. The epoxy/oxirane group is characterized by its reactivity toward both

nucleophilic and electrophilic species, and it is thus receptive to a wide range of reagents. Epoxy

monomers polymerize through step-growth and chain-growth processes. The most typical way to

polymerize epoxy monomers through step growth mechanism is the reaction with amines, which

are the most common curing agents to build up epoxy networks [36].

OHOH

OO

O O

Resorcinol

Diglycidylated ether of Resorcinol

O

-C-O-C

-OH

88

The epoxidized oil was mixed with a commercial hardener SC-15 part B (by Applied Poleramic

Inc.), composed of cycloaliphatic amine and polyoxyl-alkylamine in different weight ratios.

Previous experiments performed by this research group revealed that the best thermo-mechanical

properties were obtained when using SC-15 part B as a hardener to cure the epoxidized linseed

oil. Other tested commercial hardeners included Jeffamine T-403 and Jeffamine D-2000. The

cure reaction proceeds following a condensation procedure. The mixture was cured for 6 hrs. at

60 °C, 6 hrs. at 80 °C, 2 hrs. at 120 °C and finally 1 hr. at 180 °C. Figure 3.9 shows the evolution

of the storage modulus and the tan δ as a function of the epoxy/hardener ratio.

Figure 3.9. Storage modulus and tan δ of epoxidized linseed oil as a function epoxy/amine ratio

Basically, within the testing range, the epoxidized linseed oil thermosetting resin showed the

maximum glass transition temperature for epoxy/hardener ratios above 10/5.5, and the highest

modulus when the ratio was 10/7. An optimized Tg and modulus, trying to minimize the amount

36.8

53.0

60.5

60.8

63.067.5 68.5

70.3

0.06 0.17

0.56

0.67

0.901.08

1.15

1.42

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Tg

(C

els

ius)

Grams of hardener needed for 10 g of epoxy resin

Glass transition E' (Gpa) at 30 Celsius

Sto

rag

e M

od

ulu

s (G

Pa

)

89

of hardener, is attained when using an epoxy/amine ratio of ~10/6. Materials with low amine

proportions contain unreacted groups or even free monomers. These unreacted species are

trapped inside of the polymer network and act as plasticizers lowering both the modulus and Tg.

3.5. Preparation of the linseed oil based polyurethane.

The second approach comprised the modification of the linseed oil structure by introducing

secondary –OH groups that can be polymerized by means of a reaction with isocyanates. The

linseed oil-based polyurethane was prepared by mixing the resulting polyol with aromatic TDA.

As a first approach to evaluate the thermo-mechanical behavior of this polyurethane, a 1.0/1.1

molar ratios of the -OH group to the isocyanate group was chosen for the formulation. Using

Equation a polyol equivalent weight = 244 g/eq. was obtained. Storage modulus (E`) and tan δ

were recorded as a function of the temperature. Results revealed the presence of a crosslinked

network, with a glass transition temperature of around – 6.5°C and a modulus with a magnitude

of approx. 3 MPa at ambient conditions. This is a clear indication of the elastomeric nature of the

resulting materials, and its poor capacity to withstand deformation when stress is applied.

3.6 Polymerization of acrylated soybean oil through free radical mechanism

The third approach covered the study of the acrylated oils. Acrylated soybean oil was employed

to explore the feasibility of using oils modified with acrylate groups to generate suitable

materials for different polymer applications. The supplier states that the product has a molecular

weight of approximately 1800-7200 g/mol; and a viscosity at 25°C of 18000-32000 cps. With the

aim of improving the solubility of the radical initiator (AIBN), the acrylate soybean oil was

diluted with methyl methacrylate, a low molecular weight comonomer. DMA was also used to

90

evaluate the thermos-mechanical properties. Materials resulting from the polymerization of

acrylated soybean oil with AIBN appeared as relatively transparent, yellowish brittle and rigid

polymers. Table 3.3 lists the properties of the resulting materials.

Table 3.3. Properties of acrylated soybean oil polymers

Acrylated Soybean

Oil

(wt%)

Methyl

Methacrylate

(wt%)

E` at 30 °C

(GPa)

Tg

(°C)

n

(mol/m3)

Mc

(g/mol)

90 10 0.38 50 5246 206

85 15 0.48 55 4445 243

80 20 0.67 60 4290 251

75 25 0.75 59 3461 312

As seen in the table, was not deeply impacted by the comonomer content and ranged from 50 to

60°C. On the other hand, storage modulus was almost duplicated when adding 25 wt% of methyl

methacrylate. Higher proportions of the comonomer were not tested to minimize the presence of

commercial synthetic agents. The 25 wt% methyl methacrylate proportion allowed to adequately

dissolve the initiator and to reduce the viscosity, vastly improving the manipulation of the resin

during formulation. The rubbery moduli of the systems ranged from approximately 30 to 45

MPa, and crosslinking densities ranging from approx. 5246 to 3461 mol/m3, when increasing the

methyl methacrylate proportion from 10 to 25 wt%. The n value, as expected, decreased with the

increase in the methyl methacrylate content, due to its chain extender effect (as reflected in the

increase in the Mc values), by reducing the crosslinking density through the formation of linear

91

chains embedded in the acrylated oil network. Physical densities were around 1.08 g/cm3, and

did not statically changed as a function of the methyl methacrylate proportion

As a summary of results, so far three chemical modifications (epoxy, hydroxyl and acrylate)

have been preliminary evaluated. As seen, the step-growth process employed to formulate the

epoxy resin implies the use of a commercial amine to crosslink the epoxy monomers. This

generates a partially bio-based (around 60 wt% of the total weight of the formulation) resin that

showed a considerably better thermo-mechanical performance than the materials produced in

Chapter II, where non-modified tung oil was cationically polymerized to yield elastomeric

materials. Clearly, this modification improves the mechanical properties of oil-based materials,

yielding thermosets with modulus in the order of GPa and glass transitions temperatures around

70°C.

Concerning the second approach, the inclusion of hydroxyl groups to produce polyurethanes; it is

evident that the elastomeric nature of these resins cannot make them suitable candidates to

replace materials intended to structural or composite applications. Due to their flexibility, these

polyurethanes could be used as matrices for thermal insulation, packing or cushioning. However,

the resulting materials were extremely flexible, and lacked of mechanical resistance; therefore,

no further investigations were carried out using polyurethanes. Regarding acrylated oils

polymerized through free radical mechanism, it can be concluded that acrylated oils can be used

to make rigid thermosetting polymers and composites. These materials exhibited high

crosslinking densities, resulting in higher Tg (approx. 60°C) and improved storage modulus (~0.7

GPa) than their non-functionalized counterparts. From these findings, the epoxy and acrylate

92

moieties are promising approaches to more reactive monomers able to generate materials with

good mechanical properties than their non-modified counterparts. Epoxy resins are the main

topic on the remaining sections of this chapter, meanwhile acrylated oils will be considered in

chapter IV as a source of monomers to generate high performance materials.

It is clear however, that all of the above mentioned modifications lead to not fully, partially bio-

based resins; which still lacking of the desire properties to simulate the performance of existing

counterparts from non-renewable resources. At this point of the research, the efforts were focus

on improving the thermo-mechanical properties of the epoxy resins by introducing aromatic, low

molecular weight comonomers able to increase the chain stiffness and thus, enhancing the

mechanical performance of the oil-based epoxy resins to simulate and even to improve the

properties of commercial systems. It was also extremely important to increase the contribution of

renewable materials into the formulations, by minimizing the use of commercial compounds.

Therefore, this chapter also studies the chain-growth copolymerization (which reduces the

amount of commercial agents in the formulation per weight basis) of epoxy monomers. The

details of the copolymerization of epoxidized linseed oil and a low molecular weight aromatic

epoxy comonomer are summarized in the following sections.

3.7 Copolymerization of epoxidized linseed oil and epoxidized α-resorcylic acid through

chain-growth process

Another possible ways to polymerize epoxy monomers is through the use of small amounts of

Lewis bases that act as initiators of the anionic polymerization. Commonly used initiators

include tertiary amines such as 4-dimethylaminopyridine (DMAP). The mechanism for the

93

anionic polymerization of epoxy monomers initiated by tertiary amines is highly complex and

has not been fully described yet. Figure 3.10 depicts a simplified scheme of the anionic

polymerization of epoxy monomers.

Figure 3.10. Anionic polymerization of epoxy monomers.

In order to prepare the resins, epoxidized linseed oil and triglycidylated ether of α-resorcylic acid

(TRA) or epoxidized α-resorcylic acid, were mixed in different weight proportions. The idea of

using this aromatic trifunctional monomer lies upon the principle that the flexibility of the

polymer backbone may be markedly decreased with the insertion of groups that stiffen the chain

by impeding rotation, since more thermal energy is then required to set the chain in motion. The

presence of rigid aromatic structures confers to the materials the thermo-mechanical properties

required in their industrial and high performance applications. Dimethylamino pyridine or

DMPA was added to the mixtures in a ratio of 0.08 mol DMAP/mol epoxy. After the curing

process, the polymeric materials appeared as dark-brown stiff materials at room temperature. For

all the studied systems, suitable materials for analysis were not obtained for epoxidized linseed

oil proportions above 30 wt%; due to the lack of miscibility of the two compounds.

+ n

DMAP Epoxy Monomer

O

R R1

N+

CH3

CH3

N

O

R1

O-

R1

R

R

R = H or Alkyl Group R1

= O-Ar

NN

CH3

CH3

n

94

3.8. Scanning Electron Microscopy

The fracture surfaces of the thermosets were studied by scanning electron microscopy (SEM).

The results of the morphological study are shown in Figure 3.11. The 100 wt% α-epoxy

resorcylic acid formulation possesses a fracture surface and an internal structure of a brittle

material, characterized by a highly glassy surface; where crack marks, ridges, and planes are

dispersed all over the surface [37]. Regarding the 100 wt% epoxy linseed oil resin, it shows the

morphology of a ductile material, with a smooth and featureless surface denoting a highly plastic

deformation [38]. The 80-20 and 70-30 wt% copolymers present a very irregular surface,

characterized by randomly distributed cavities, possibly denoting the presence of a non-

homogeneous structure, through the formation of rubbery domains as a result of micro-phase

separation of the components.

3.11.a

95

Figure 3.11. SEM micrographs of the fracture surface of 100 % α-RA (3.11.a), 80 % α-RA – 20 %

ELO (3.11.b), 70 % α-RA – 30 % ELO (3.11.c), and 100 % ELO (3.11.d).

3.11.b

3.11.c

3.11.d

96

3.9. Thermo-mechanical characterization of the epoxidized linseed oil (ELO) and

epoxidized α-resorcylic acid (ERA) copolymers.

Figure 3.12 shows the temperature dependence of the storage modulus (E`) and the tan δ. The

peak in tan δ corresponds to the main mechanical relaxation of the matrix, which was used as a

criterion to determine the Tg of the polymer.

Figure 3.12. Temperature dependence of the storage modulus (3.12.a) and loss factor or tan δ

(3.12.b) as a function of the composition of the linseed oil – α-RA epoxy resins

100% ERA

90% ERA- 10% ELO

80% ERA - 20% ELO

70% ERA - 30%

ELO

100% ELO

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

-50 -25 0 25 50 75 100 125 150 175 200

Sto

rag

e M

od

ulu

s (P

a)

Axis Title

100% ERA90% ERA - 10%

ELO

80% ERA - 20%

ELO

70% ERA - 30%

ELO

100% ELO

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

-50 -25 0 25 50 75 100 125 150 175 200

Ta

n δ

Temperature (°C)3.12.b

3.12.a

97

Results revealed that the materials behaved as thermosets. As seen in the curves, the storage

modulus initially remained approximately constant. As the temperature increased, the storage

modulus exhibited a drop over a wide temperature range (glass transition), followed by a

modulus plateau at high temperatures, where the material behaved like a rubber. This region,

called the rubbery plateau, evidences the presence of a stable cross-linked network. The modulus

drop is associated with the beginning of segmental mobility in the cross-linked polymer network;

whereas the constant modulus at temperatures above 150 °C (for the systems containing

triglycidylated α-RA) and above 50 °C (for 100 wt. % epoxidized linseed oil) is the result of the

cross-linked structure of the polymer. As can be seen, the transition between the glassy to the

rubbery state was relatively sharp and began at around 75 °C for the tested materials containing

triglycidylated α-RA; and started below room temperature for the 100 wt. % epoxidized linseed

oil resin. Table 3.4 summarizes the main properties of the bio-based epoxy thermosets.

Table 3.4. Mechanical properties of the triglycidylated α-RA and epoxidized linseed oil epoxy

resins

Triglycidylated ether

of α-resorcylic acid

(wt%)

Epoxidized

linseed oil

(wt%)

E` at 30 °C

(Pa)

Tg

(°C)

Density of active

chains (mol/m3)

100 0 3.78E+09 113.7 7176

90 10 3.09E+09 107.9 2701

80 20 2.22E+09 116.2 1808

70 30 1.40E+09 116.5 1520

0 100 1.64E+06 20.5 102

98

As shown, the storage modulus (E’) at room temperature considerably decreased when the

triglycidylated α-RA wt. % content was reduced. The decline in the modulus values is directly

correlated to the decrease in the density of active chains or n; parameter that is often associated

to the crosslinking density of the material. For the calculation of n, it was assumed that all of the

components in the systems entered into the network, which is in reality impossible. However,

after the Soxhlet extraction of samples with acetone for 6 hours, it was observed that the total

amount of monomers that did not polymerize ranged from 14 % to 19 % (from epoxidized

resorcylic acid 100 wt% to epoxidized linseed oil 100 wt%). The glass transition temperature

was not highly influenced by the reduction in the α-RA wt. % content and it showed its

maximum value for the 80 epoxidized α-RA wt. % - 20 wt. % epoxidized linseed oil systems. As

expected, the poorest mechanical properties where exhibited by the 100 % wt. epoxidized LO

system. It is of worth to highlight that there are two opposite mechanical behaviors present in the

two epoxy monomers. On the one hand, resins containing α-resorcylic acid present the typical

behavior of glassy, brittle polymers, with modulus in the order of GPa at room temperature and

glass transition temperatures above 100°C. On the other hand, the linseed oil proves to be

completely elastomeric and ductile material, having a crosslinking density even lower than a

common rubber band (typically n = 190 mol/m3 [39]).

As previously stated, suitable materials for analysis were not obtained for epoxidized linseed oil

proportions above 30 wt%. As a result of this observation, concerns about the miscibility of the

two phases arose during the characterization of the resulting materials. In order to study this

phenomenon, the correlation between the glass transition temperature and the composition,

through the Fox model was employed to evaluate the miscibility of the phases. This model

99

pertains to random miscible systems, with weak intermolecular interactions, and it is commonly

used as a criterion to evaluate the homogeneity and miscibility between the components [40].

Figure 3.13 shows the correlation between the expected values and the experimental data,

denoting the regions where one or two phases were visibly evident. Fox model predicted a

decrease in the Tg when the triglyceride-based epoxy resin was mixed with the epoxidized α-

resorcylic acid. As seen in the plot, correlation was good when 10 wt% of linseed oil was added

to the α-resorcylic acid. However, when the proportion was increased to 20 and 30 wt%, the

model underestimated the values; and higher glass transitions were observed. It is evident that

even at relatively low proportions of linseed oil, deviations from the Fox equation were

observed, indicating that the miscibility of the phases was not accomplished.

Figure 3.13. Relationship between 1/Tg and relative epox. α-resorcylic acid weight fraction

As a second approach, the correlation between the solubility parameters (δ) of the compounds

was evaluated. A criterion of good solubility for a given binary system is achieved when δ1 ~ δ2.

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1 /

Tg

(K

-1)

Epox. α-resorcylic acid weight fraction

Fox Equation

Experimental data

one-two phases region

One phase Region

Two phase Region

100

It provides a useful practical guide for non-polar systems [41]. As an approximation, the reported

solubility parameters of a bisphenol A type epoxy resin and linseed oil have been considered

[42,43]. In the case of the epoxy resin, the influence of the dipolar interactions, as well as

hydrogen bonding, can been neglected; thus reducing the Hansen three component model to only

the dispersion contribution. Therefore, approximating δD ~ δt for non-polar compounds, the

reported value of the solubility parameter for a Bisphenol A type resin is 20.0 MPa1/2. On the

other hand, the solubility parameter for linseed oil is 16.4 MPa1/2. A difference of around 3.6

units in this parameter indicates that the linseed oil is sparingly soluble in the epoxy resin, which

structurally resembles the epoxidized derivative of α-resorcylic acid.

It is also important to discuss aspects related to the structural characteristics of the monomers,

which highly influence the reaction. On the one hand, epoxidized oils contain oxirane

functionalities that are located near the middle of the 18-carbon length fatty acid chains. As a

consequence of this feature, these groups are sterically hindered at both carbons; heavily

diminishing their reactivity. On the other hand, the triglycidylated ether of α-resorcylic acid

contains pending oxirane functionalities that are non-sterically hindered, facilitating the attack of

the anionic growing species generated during the propagation stage of the reaction. Mechanistic

studies indicate that initiation of the reaction involves SN2 displacement to form alkoxide ion,

and the propagation occurs via nucleophilic displacement involving the new alkoxide ion [44]. It

is well known that the SN2 transition state involves partial bond formation between the incoming

nucleophile and the substrate. It seems reasonable to consider that a hindered, bulky substrate

prevents the approach of the nucleophile, making bond formation difficult. As a result of this

steric effect, during the polymerization stage, the reaction might have had more affinity for the

101

epoxy monomers derived from the triglycidylated ether of α-resorcylic acid; than for the epoxy

monomers obtained from the modification for the linseed oil. After the curing and subsequent

formation of a three-dimensional network, the long chain rubbery molecules of the triglyceride

derived epoxy resin can either participate in the phase separation (forming small rubbery

domains), or taking part in the epoxy matrix by chemical bonding [45]. This process might

happen randomly, fact that is reflected in the lack of tendency observed in the evolution of the

glass transition temperature as a function of the linseed oil weight fraction.

Recalling from the discussion about the thermo-mechanical properties and the resulting effect of

adding a low molecular weight aromatic comonomer, it is very clear that the presence of rigid

aromatic structures do confer to the materials the typical behavior of glassy, brittle polymers; by

providing a highly crosslinked structure, able to withstand deformation when stresses are

applied. Compared to the triglyceride-based epoxy monomers, aromatic epoxy monomers

provide a considerably better option to formulate materials with good thermo-mechanical

performance.

In order to compare the results obtained in this research, the triglycidylated ether of α-resorcylic

acid was mixed in different weight proportions with the well-known commercial epoxy resin

EPON 828. This is an undiluted clear di-functional bisphenol A/epichlorohydrin derived liquid

epoxy resin. It is synthesized following a similar chemical approach to the one used in this

research to attach epoxy groups to the α-resorcylic acid. The triglycidylated ether of α-resorcylic

acid (TRA) and the EPON 828 were mixed in different weight proportions, and were also cured

using DMAP as an initiator.

102

Figure 3.14 shows the micrographs of the fracture surfaces. All the resins present homogeneous

morphology with no visible phase domains. The surfaces are characterized by smooth, glassy

microstructures, without any plastic deformation.

3.14.a

a

3.14.b

103

Figure 3.14. SEM micrographs of the fracture surface of 100 wt% α-RA (3.14.a), 75 wt% α-RA – 25

wt% EPON 828 (3.14.b), 50 wt% α-RA – 50 wt% EPON 828 (3.14.c), and 100 wt% EPON 828

(3.14.d).

3.14.c

3.14.d

104

Concerning their thermo-mechanical behavior, Table 3.5 summarizes the main properties these

epoxy thermosets.

Table 3.5. Mechanical properties of the triglycidylated α-RA and EPON 828 epoxy resins

Triglycidylated ether of

α-resorcylic acid (wt%)

EPON 828

(wt%)

E` at 30 °C

(GPa)

Tg

(°C)

Density of active

chains (mol/m3)

100 0 3.78 114 7176

75 25 4.13 115 3854

50 50 2.43 124 2065

25 75 3.24 115 2531

0 100 2.40 140 2732

Results revealed that the properties of the mechanical properties of the triglycidylated α-RA

based resin are similar to those shown by their commercial counterpart, exhibiting moduli values

in the order of Giga Pascal at room temperature and glass transition temperatures above 100 °C.

This finding suggests that the density of active chains plays a key role in the final properties of

the materials, especially in the magnitude of the modulus, a physical measurement of their

stiffness. It is very clear that the modulus is strongly dependent of the variation of this parameter.

Therefore, when synthesizing thermosetting materials, it is of extreme importance to provide the

monomers with structural characteristics that ensure the maximization of the number of active

polymer chains forming the crosslinked network. One of the ways to achieve high crosslinking is

by means of increasing the number of functionalities of the monomers. It is well known that as

the functionality increases, the network crosslinking density increases, thus increasing the

thermo-mechanical properties [46].

105

In order to have a better understanding of the effect of the functionality on the final properties of

the networks, the diglycidylated ether of resorcinol and the triglycidylated ether of α-resorcylic

acid (di and tri functional epoxy aromatic monomers, respectively) were copolymerized through

chain growth polymerization employing DMPA as initiator. As previously stated, from the point

of view of the polymer growth mechanism, two different entirely processes, step and chain

polymerization, are distinguishable. For both mechanisms, if one of the reactants has

functionality higher than 2, branched molecules and an infinite structure can be formed;

otherwise a linear thermoplastic material is obtained [47]. Table 3.6 shows the obtained results,

regarding their response to an applied cyclical stress.

Table 3.6. Thermo-mechanical properties of the diglycidylated ether of resorcinol and

triglycidylated α-resorcylic acid copolymers

Diglycidylated

ether of

resorcinol (wt%)

Triglycidylated ether

of α-resorcylic acid

(wt%)

E` at

30 °C

(GPa)

Tg

(°C)

E` at Tg +

30 °C

(GPa)

Density of

active chains

(mol/m3)

100 0 3.15 132.49 0.125 11504

75 25 3.04 153.94 0.162 14210

50 50 3.41 168.86 0.149 12656

25 75 4.11 180.19 0.353 29281

0 100 2.90 212.51 0.433 33666

106

Figure 3.15 shows the temperature dependence of the storage modulus (E`) and the tan δ. The

peak in tan δ corresponds to the main mechanical relaxation of the matrix, which was used as a

criterion to determine the Tg of the polymer.

Figure 3.15. Temperature dependence of the storage modulus (3.15.a) and loss factor or tan δ

(3.15.b) as a function of the composition of the resorcinol weight content for the resorcinol-α-

resorcylic acid epoxy resins.

1.E+07

1.E+08

1.E+09

1.E+10

30 55 80 105 130 155 180 205 230 255 280 305 330

Log

Sto

rag

e M

od

ulu

s (P

a)

Temperature (°C)

100% Resorcinol

75% Resorcinol

50% Resorcinol

25% resorcinol

0% Resorcinol

0.0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200 250 300 350

Ta

n δ

Temperature (°C)

100% Resorcinol

75% Resorcinol

50% Resorcinol

25% Resorcinol

0% Resorcinol

3.15.b

3.15.a

107

Considering the modulus at room temperature, the values are approximately constant (~ 3- 4

GPa), and are just a weak function of the weight ratio of the two monomers. This behavior is not

surprising, since the modulus for glassy polymers just below the glass transition temperature is

known for being approximately constant over a wide range of polymers; having a value of

around 3 x 109 Pa. In this state, molecular motions are restricted to vibrations and short-range

rotational motion. Classic explanations for this phenomenon imply the use of Lennard-Jones

potentials, as well as carbon-carbon bonding force fields [39]. Regarding the glass transition, it is

evident that is a strong function of the proportion of the monomers, as well as their chemical

functionality, producing a difference of approx. 80°C when switching from two to three epoxy

groups (di and tri glycidylated monomers). The rapid, coordinated molecular motion in this

region is governed by principles of reputation and diffusion. This phenomenon can be suppressed

or retarded (in terms of energy), if the crosslinking density is increased. As seen in the Table, the

higher the crosslinking density, the more constrained the chain motion; and thus the stiffer and

more thermally resistant the molecular structure is, fact that is reflected by the values of the

rubbery modulus (or E` at Tg + 30°C). Clearly, increasing crosslinking density approx. 3 times

led to an increase of around 80°C in the Tg. As the temperature is raised past the rubbery plateau

region for linear amorphous polymers, the rubbery flow region is reached. It has to be

emphasized that this region does not occur for crosslinked polymers. This behavior was expected

for the 100 wt% diglycidylated ether of resorcinol, however it was not clearly observed in the E`

vs temperature plots. After raising the temperature past the 250°C, the modulus dropped and

remained unstable, probably due to thermos-oxidative decomposition of the polymers.

Differential scanning calorimetry studies revealed signs of thermal decomposition for all of the

materials when approaching 300°C.

108

4. Conclusions

This chapter focuses on the chemical modification of two different functional groups of biomass

derived materials; the carbon-carbon double bonds of triglycerides, and the hydroxyl groups of

phenolic compounds. These chemical modifications were addressed to introduce more reactive

functional groups able to polymerize and to produce materials with a better thermo-mechanical

performance. Three chemical modifications (epoxy, hydroxyl and acrylate) were preliminary

evaluated. The step-growth process employed to formulate the epoxy resin implies the use of a

commercial amine to crosslink the epoxy monomers. It was observed that the epoxidized oil,

when mixed in an epoxy/amine ratio of ~10/6, produced resins with acceptable thermo-

mechanical properties, showing a Tg of around 70°C and a modulus of around 1.4 GPa at room

conditions. Regarding acrylated oils polymerized through free radical mechanism, it can be

concluded that acrylated oils can be used to make rigid thermosetting polymers and composites.

These materials exhibited high crosslinking densities, resulting in higher Tg (approx. 60°C) and

improved storage modulus (~0.7 GPa) than their non-functionalized counterparts. These two

approaches produced materials with considerably better thermo-mechanical performance than

the thermosets produced in chapter II.

This chapter also studied the chain-growth copolymerization (which reduces the amount of

commercial agents in the formulation per weight basis) of epoxy monomers. This approach

allowed increasing the contribution of renewable materials into the formulations, by minimizing

the use of commercial crosslinking compounds. Likewise, efforts were focus on improving the

thermo-mechanical properties of the epoxy resins by introducing triglycidylated ether of α-

resorcylic acid, a low molecular weight comonomer able to increase the chain stiffness and thus,

109

enhancing the mechanical performance of the oil-based epoxy resins. Testing showed that this

aromatic monomer is able to produce resins with high Tg and modulus (~ 114°C and 3.8 GPa).

When combined with epoxidized oil, a decrease in the storage modulus when higher amounts of

epoxidized linseed oil were added to the mixtures was observed. Tg appeared to be not strongly

dependent of the amounts of epoxidized linseed oil. SEM images confirmed the glassy brittle

nature of the materials and revealed possible thermodynamic incompatibility of these two

compounds. When compared to samples of commercial origin, testing showed the mechanical

properties of the triglycidylated α-RA based resins are similar to those shown by EPON 828

epoxy resin. Studies about effect of the functionality on the final properties of the networks,

employing the diglycidylated ether of resorcinol and the triglycidylated ether of α-resorcylic acid

as models, revealed that the crosslinking density rises as a functionality of the monomers

increases, thus increasing the thermo-mechanical properties.

110

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116

CHAPTER IV

Synthesis and Characterization of Interpenetrating Polymer Networks (IPNs) Using

Biomass Derived Materials.

1. Introduction

Thermosetting polymers are generally stronger than thermoplastic materials due to the presence

of a three-dimensional network of bonds (or cross-linking). This structural characteristic makes

them suitable for high-temperature applications, showing a number of advantages over

thermoplastics, such as elevated heat distortion temperatures, good solvent resistance and high

modulus. These outstanding properties have resulted in their use in a wide range of commodity,

engineering and specialty applications [1].

Nowadays, there is an increasing growing interest to develop novel bio-based products. The

synthesis of bioplastics, which are intended to simulate the performance of existing counterparts

from non-renewable sources, also goes hand in hand with the improvement of the current

available materials; most of them traditionally produced from petroleum based monomers. To

expand the physical and chemical performance of polymeric materials, multicomponent

macromolecular systems have been proposed and extensively used for several decades [2].

Amongst several type of systems, interpenetrating polymer networks or IPNs are a particular

class of hybrid polymer consisting of two (or more) crosslinked polymers, in which the

macromolecular networks are held together by permanent entanglements with only few covalent

117

bonds between the polymers [3]. IPNs are an intimate combination of two or more polymers in

network form, where at least one of the polymers is polymerized and/or crosslinked in the

immediate presence of the other [4].

Ideally, IPNs interlace or interpenetrate on a molecular scale; however, the actual

interpenetration process may be limited by phase separation. When two polymers are mixed, the

most frequent result is a system that exhibit almost total phase separation. From a

thermodynamical point of view, this phenomenon arises as a result of the reduction on the

combinatorial entropy of mixing, compared to the enthalpic contribution of the mixing process,

of the two types of polymer chains [4]. However, if mixing is accomplished simultaneously with

crosslinking, phase separation may be kinetically controlled by permanent interlocking of

entangled chains [3]. Also, in recent years, with the aim of minimizing phase separations, there

has been an increasing interest in developing graft-IPNs, in which the two networks are

chemically cross-linked [5]. These multicomponent polymer systems seek to combine the best

properties of two or more different polymer networks in order to achieve a material with better

properties compared to their not-interpenetrated counterparts. It has been found that the network

interlock has measurable influences on the curing behaviors and on the mechanical properties of

the resulting materials [6].

Over the years, IPNs have been categorized into different ways, mainly according to their

synthesis method and also by their structure. A detailed description is given by Sperling [7],

however, the description provided in this document will principally cover the synthesis method,

which is the main focus of this research. According to the chemistry of preparation, IPNs can be

118

classified into: (i) Simultaneous IPN, when the precursors of both networks are mixed and the

two networks are synthesized at the same time by independent, non-interfering reactions. (ii)

Sequential IPNs, in which polymer network I is synthesized first and then monomer II, plus

crosslinking agents and catalyzers, are swollen into network I and polymerized in situ. If a

crosslinker is present, fully IPN results, while in the absence of a crosslinker, a network having

linear polymer embedded within the first network is formed (semi-IPN) [8].

In the last decades, few research groups have focused their work on the synthesis and

characterization of IPNs from natural products, beginning with the pioneering work of Sperling

and his coworkers, who developed IPNs from castor oil-based elastomers and styrene monomers

[9]. This fact stands up as a key finding, giving the opportunity to perform research on a field

lacking of recent developments. Also, other naturally functionalized triglyceride oils and their

use in producing semi-IPNs is reviewed by the same group [10]. After this work, other groups

have used castor oil as a source of polyurethanes to prepare oil-based IPNs [11,12]. Other oil-

based IPNs include the use of oligomerized soybean oil [13], tung oil [14], and canola-oil based

polyol with primary terminal functional groups (ref semi and full IPN). It is of worth to highlight

that most of these oil-based IPNs structures are not completely, but partially, derived from

biological raw materials and fossil fuels. IPNs between commercial resins, such as epoxy, and

elastomeric materials of natural origin are known in literature. Examples include acrylate and

methacrylate monomers derived from vernonia oil combined with bisphenol A-type resin [15],

flexible methacrylate network based on camelina oil and a rigid aliphatic glycidylether modified

bisphenol A based liquid epoxy resin [16].

119

On the other hand, lignin, which annual production on earth has been estimated to be in the range

of 5 – 36 x 108 tons, is undoubtedly the most promising natural source of aromatic monomers for

the chemical industry. For years, synthesis strategies addressed to modify lignins have struggle

with low reactivity associated to steric hindrance [17]. However, some novel technologies based

on biomass refinery appear as a good alternative to overcome this problem. Modern pyrolysis

techniques stand as a sustainable way for aromatics biosourcing, by providing low molecular

weight fragments of natural lignin, which can be used as platform chemical for organic synthesis.

A vast amount of lignin-based compounds, including phenol and more complex guaiacol and

syringol derivatives, has been identified in pyrolysis oils [18–20]. Tannins have also been

explored as a source of aromatic macromonomers to produce thermosetting resins [21]. To the

best knowledge of the authors of this project, interpenetrated polymer systems employing

monomers, or their precursors, that can be potentially obtain from biorefinery have not been

reported in literature.

In this Chapter, the formulation of interpenetrating polymer networks, comprising epoxy and

acrylate phases, is extensively studied; mainly focusing on the thermo-mechanical properties of

the resulting materials (Figure 4.1). Two types of compounds were tested as the epoxy phase of

the system, epoxidized linseed oil (containing fatty acid chains bearing epoxy moieties) and

glycidyl ether of α-resorcylic acid (or epoxidized α-resorcylic acid), crosslinked with

commercial multifunctional amines. The acrylate phase was synthetized using acrylated soybean

oil, slightly diluted with methyl methacrylate. As seen in Chapter III, the copolymerization of

epoxidized vegetable oil with epoxidized α-resorcylic acid vastly improved the thermo-

mechanical properties of the materials. However, phase separation occurred for mixtures

120

containing epoxidized triglycerides in weight contents higher than 30 wt%. The idea of

developing an interpenetrated system containing an epoxy phase derived from α-resorcylic acid,

and an acrylate phase derived from vegetable oil arises as an approach to not only improve the

properties by including an aromatic monomer in the system, but also to minimize the phase

separation phenomena by kinetically controlling this thermodynamical process by permanent

interlocking of entangled chains, avoiding phase separation.

Figure 4.1. Generic scheme of the IPNs containing epoxy and acrylate networks.

The strategy to prepare the IPNs can be divided into two sections. The first one comprises the

chemical modification of the raw materials, as well as a quantitative analysis of the targeted

functional groups. In a second step, the modified monomers are polymerized and the resulting

materials characterized, mainly focusing on their thermo-mechanical behavior. α-resorcylic acid

(a phenolic acid with two hydroxyl and one carboxylic acid functional groups) was modified

CH2O

CH3

CH3

O

O

O

O

O

R

O

O

R

CH3

OH

O O

CH2

O

O

O

O

O

R O

R

CH3

O O O

O O

O O

O

O

O

Epoxy Phase

Acrylate Phase

Acrylated Soybean Oil Methyl

Methacrylate

Epoxidized Linseed Oil Epoxidized α-Resorcylic Acid

+

OR

IPN

* Crosslinked with commercial amine curing agents

* Polymerized with AIBN

121

with reactive epoxy moieties, and used as low molecular weight comonomer to enhance the

density of aromatic structures in the crosslinked network. From the literature review, it was

found that most of the current IPN are not fully, but partially, prepared from biological raw

materials and fossil fuels. The novelty of the present study is not only derived from the selection

of the macromolecular components, which try to provide a response for the need of IPNs

prepared from formulations including a higher contribution of biological materials; but also from

the final properties that can be generated when associating the phases in the IPN structure.

2. Experimental Section

2.1 Materials

Linseed oil (LO) was purchased from VWR International (USA) and used with no further

purification. Glacial acetic acid (99.5 %), hydrogen peroxide (30 %), sodium carbonate, toluene,

petroleum ether, ethyl acetate, iodine monochloride, Wijs’ solution, 33 % HBr in glacial acetic

acid, methyl methacrylate, 2,2′-Azobis(2-methylpropionitrile) (AIBN), and Amberlite ion-

exchange resin were all AR grade and also acquired from VWR International (USA). α-

Resorcylic acid (α-RA, ≥ 98%), epichlorohydrin (99.0 %), sodium hydroxide (≥ 98%), and

dichloromethane were purchased from VWR (US). Benzyltriethylammonium chloride (≥ 98%),

used as a phase transfer agent, was also acquired from VWR (US).

Acrylated soybean oil (Figure 2) was purchased from Sigma Aldrich (USA). In Figure 4.2, R

represents the fatty acid chain modified with the acrylate group. The supplier states that the

product has a molecular weight of approximately 1800-7200 g/mol. The average number of

acrylate groups per triglyceride has not been determined by the supplier or Sigma. However, it

122

O

O

O

O

R

O

O

R

CH3

OH

O O

CH2

has been established that it contains in average 4.5 double bonds, that can be first converted into

epoxy groups, and then into acrylate moieties [22].

Figure 4.2. Chemical structure of the acrylated soybean oil.

2.2 Synthesis procedures

2.2.1 Synthesis of epoxidized triglycerides

Refer to Chapter III for the synthesis of epoxidized linseed oil.

2.2.2 General procedure for the glycidylation of α-resorcylic acid.

Refer to Chapter III for the glycidylation of α-resorcylic acid.

2.2.3 Synthesis of IPNs

As previously detailed, IPNs can be prepared according to two different basic synthesis methods,

known as Simultaneous IPNs (SIN) and as Sequential IPNs. During the simultaneous approach,

the monomers or polymer constituents, plus crosslink agents and catalyzers are mixed together.

After mixing, the two polymerization reactions are carried out simultaneously as indicated by its

name. In terms of the chemical mechanism they follow to polymerize, there is no interference

whatsoever between reactions. In the case of sequential method, polymer network I is

123

synthesized first and then monomer II plus crosslinking agents and catalyzers are swollen into

the network I and polymerize in situ. For this work, the simultaneous method was chosen to

prepare the IPNs. Two types of IPNs were synthesized and studied, both of them composed of

two different phases, the epoxy phase and the acrylate phase. The epoxy phase is polymerized

through a step-reaction polymerization, by curing the epoxy monomers with a commercial

amine. The acrylate phase, on the other hand, is polymerized through a chain reaction that

requires an initiator to generate reacting species and then propagate the reaction. The types of

IPNs are detailed as follows.

System A: Epoxidized linseed oil/acrylated soybean oil IPNs

The first type of IPN consisted of combining triglyceride-derived materials, the epoxidized

linseed oil (polymerized through step-growing mechanism) and the acrylated soybean oil

(polymerized through chain-growth mechanism). First, the epoxy phase was prepared by mixing

epoxidized linseed oil with the commercial hardener SC-15 part B (Applied Poleramic Inc.),

composed of cycloaliphatic amine and polyoxyl-alkylamine, in a 10/6 weight ratio. Even though

technical information for this agent is very limited, previous experiments performed by this

research group revealed that the best thermo-mechanical properties were obtained when using

SC-15 part B as a hardener to cure the epoxidized linseed oil (refer to Chapter III).

The components of the acrylated phase, acrylated soybean oil, methyl methacrylate (added to

reduce the high viscosity of the acrylate oil in a 20 wt% of the total weight of acrylated oil) and

2,2-azobis(2-methylpropionitrile) or AIBN as initiator (1 wt% of the total weight of the acrylate

phase) were mixed separately and then added to the epoxy phase precursor solution in different

124

weight percentages. After mixing these solutions, all samples were placed in glass moulds and

then cured in the oven following the next heating sequence: 50°C for 1 hr., 60°C for 1 hr., 75°C

for 2 hours, 120°C for 12°C hours, and finally 180°C for 1 hour.

System B: Epoxidized α-resorcylic acid/acrylated oil IPNs

The second type of IPN consisted of combining a phenolic-based epoxy resin with the

triglyceride-based acrylate resin. The epoxy phase was prepared by mixing the triglycidylated

derivative of the α-resorcylic acid (TRA) or epoxidized α-resorcylic acid with the commercial

hardener Jeffamine T-403, a well-known trifunctional amine, in a 10/7 weight ratio. The

acrylated phase is exactly as the one described for above for the type A IPN. After mixing these

solutions, all samples were also placed in glass moulds and then cured in the oven following the

next heating sequence: 50°C for 1 hr., 60°C for 1 hr., 75°C for 2 hours, 120°C for 12°C hours,

and finally 160°C for 1 hour.

2.3. Characterization of Materials and Networks

2.3.1 Fourier Transform Infrared Spectroscopy (FTIR)

With the aim of studying the chemical functionalization of both, the linseed oil and the α-

resorcylic, FTIR was performed. Infrared spectra of the starting materials, as well as the

epoxidized products, were measured by attenuated total reflection (ATR) method using a

Thermo Nicolet 6700 Fourier transform infrared spectrometer connected to a PC with OMNIC

software analysis. All spectra were recorded between 400 and 4000 cm-1 over 64 scans with a

resolutions of 16 cm-1.

125

2.3.2 Kinetics of Gelation

The characteristic peaks in the infrared region for the acrylic unsaturation (1638 cm-1) and epoxy

ring (915 cm-1) respectively were used to calculate the conversion of the acrylic and epoxy

groups. The band at approx. 2960 cm-1, assigned to the C-H stretch absorption, was used as a

reference because it does not suffer any chemical modification during the process. The progress

of curing reaction was evaluated using Equation (4.1):

]��^V�_��� (%) =abbbc1 −

(G���G� e)!f

(G���G� e)1f

ghhhi

∗ 100 (4.1)

Where, Apol = 915 cm-1 or 1638 cm-1 peak areas, Aref = 2960 cm-1 peak area, t = reaction time, and

0 = initial moment of reaction (uncured mixture) [23].

2.3.3 Determination and Measurement of the Extent of Reaction at the Gel Time.

Gel time was determined following the method described by the ASTM D2471-99 [24]. This test

method covers the determination of the time from the initial mixing of the reactants of a

thermosetting plastic composition to the time when solidification commences. For each system,

at least three sets of experiments were performed. The extent of the reaction at the gel point can

be calculated by means of two theories, and is of especial interest. Two theories were compared.

According to Carothers [25], the critical extent of the reaction, Pc, at the gel point is given by

Pc = 2 / favg (4.2)

where, favg represents the average functionality of the system, and is given by

favg = ∑Nifi / ∑Ni (4.3)

and where, Ni is the number of molecules of monomer I with functionality fi.

126

Flory and Stockmayer [26], developed another relationship, correlating the extent of conversion

at the gel point for stoichiometric mixtures,

Pc = 1 / [(fw,epoxy -1 ) (fw,amine – 1) (4.4)

Where, fw,epoxy represents the weight average functionality of the epoxy monomers, and fw,amine

represents the weight average functionality of the amine monomers.

2.3.4 Soxhlet extraction.

Soxhlet extraction was employed to characterize the structures of the bulk copolymers following

the ASTM D5369-93 [27]. An appropriate amount of the bulk polymer was extracted for 24 h

with 200 mL of refluxing dichloromethane with a Soxhlet extractor. Upon extraction, the

resulting solution was concentrated by rotary evaporation and subsequent vacuum drying. The

soluble substances were isolated for further characterization. The insoluble solid was dried in a

vacuum oven for several hours before it was weighed.

2.3.5 Scanning Electron Microscopy (SEM)

SEM study was performed with the purpose of obtaining a topological characterization of the

materials. The fractured surfaces of pre-chilled samples in liquid nitrogen were studied by

scanning electron microscope (Zeiss EVO 50 variable pressure scanning electron microscope

with digital imaging and EDS). Samples were sputtered with gold prior to SEM observations

with an EMS 550X auto sputter coating device with carbon coating attachment.

127

2.3.6 Thermal Properties

The thermal properties of the IPNs, as well as the parent resins, were investigated by Differential

Scanning Calorimetry (DSC) on a TA DSC Q2000. Nitrogen was used as the purge gas and the

samples were placed in aluminum pans. For each material sample, the thermal properties were

recorded at 5°C/min. in a heat/cool/heat cycle from 30°C to 300°C.

2.3.7 Thermo-mechanical properties of the networks.

Dynamic mechanical analysis (DMA) on a TA Instruments RSA III was carried out to assess the

thermo-mechanical properties by three-point bending. The tests were performed at temperatures

ranging from 25 to 250°C with a heating rate of 10°C/min. The frequency was fixed a 1Hz and a

sinusoidal strain-amplitude of 0.1 % was used for the analysis. The dynamic storage modulus

(E′) and tan δ curves were plotted as a function of temperature. To be able to explain the effects

of the co-monomers on the mechanical properties, it is useful to calculate the crosslinking

density (n, mol/m³). The crosslinking density can be estimated from the experimental data using

the rubber elasticity theory. Thermosets behave as rubbers above Tg. At small deformations,

rubber elasticity predicts that the modulus storage (E’), of an ideal elastomer with a network

structure is proportional to the crosslinking density according to Equation (4.5) [28]:

�� = 3��� = 3��/�� Equation (4.5)

where E` is the rubbery storage modulus, R is the gas constant (8.314 J/mol K), T is the absolute

temperature (K), ρ is the density of the sample (g/m³), and Mc is the molecular weight between

crosslinks (g/mol). The temperature and rubbery modulus were determined for the calculation of

the equation at Tg + 30°C. The temperature at which the peak of the tan delta presents a

maximum was considered as the glass transition temperature of the material.

128

2.3.8 Linear Elastic Fracture Mechanics

In order to characterize the fracture toughness of the graft-IPNs synthesized, in terms of the

critical stress intensity factor, KIC, quasi-static fracture tests were performed following the

method described by the ASTM D5045-14 [29]. The cured IPN sheets were machined into

rectangular coupons of dimensions 70 mm x 20 mm and 2.8 mm thickness. An edge notch of 3

mm in length was cut into the samples, and the notch tip was sharpened using a razor blade. An

Instron 4465 universal testing machine was used for loading the specimen in tension and in

displacement control mode (crosshead speed = 1mm/min). The load-deflection data was recorded

up to crack initiation and during stable crack growth, if any. The crack initiation toughness or

critical stress intensity factor, KIC, was calculated using the load (F) recorded at crack initiation.

For each system, at least three sets of experiments were performed at laboratory conditions. The

mode-I stress intensity factor for a single edge notched (SEN) tensile strip using linear elastic

fracture mechanics is given by Equation (4.6) [5],

Equation (4.6)

where,

Equation (4.7)

and a is the edge crack length, w is the specimen width, B is the specimen thickness and F is the

peak load. All data are expressed as means ± standard deviations. Statistical analysis was

performed by one-way analysis of variance (ANOVA) in conjunction with Tukey`s post hoc test

for multiple comparisons using Minitab 17 software.

Ic

F a aK f

Bw w

π =

2 3 4

1.12 0.23 10.6 21.7 30.4a a a a a

fw w w w w

= − + − +

129

The critical energy release rate, GIc, can be related to the stress-intensity factor, KIc, in plane

strain conditions by Equation (4.8) [30]:

kᵢ� = (mᵢ))�

F (1 − ^F ) Equation (4.8)

where, v is the Poisson`s coefficient and E the Young`s modulus of the material. For all

formulation, v, was taken equal to 0.33 [31].

3. Results and Discussion

3.1 Chemical Functionalization of Raw Materials

3.1.1 Synthesis of Epoxidized Triglycerides

Refer to Chapter III for details about the synthesis of epoxidized linseed oil.

3.1.2 Functionalization of phenolic compounds with epoxy groups. Synthesis of

triglycidylated ether of α-resorcylic acid

Refer to Chapter III for details about the glycidylation of α-resorcylic acid.

3.2. Characterization of the IPNs.

3.2.1 System A: Epoxidized linseed oil/acrylated soybean oil IPNs

3.2.1.1 Scanning Electron Microscopy

Scanning electron microscopy was used to investigate the morphology of the systems. Figure 4.3

provides the obtained images of the fracture surfaces of the IPNs as well as the parent resins, pre-

chilled in liquid nitrogen.

130

4.3.a

4.3.b

4.3.c

131

Figure 4.3. SEM micrographs of the fracture surface of 100 % epoxidized linseed oil (4.3.a), IPN

80% epoxy - 20% acrylate (4.3.b), IPN 60% epoxy - 40% acrylate (4.3.c), IPN 40% epoxy - 60%

acrylate (4.3.d), IPN 20% epoxy-80% acrylate (4.3.e), 100% acrylated soybean oil (4.3.f).

4.3.d

4. 3.e

4.3.f

132

As shown, the fracture surfaces of the parent resins are characterized by smooth, glassy and

homogeneous microstructures; without any sign of plastic deformation [32,33]. The parent

materials exhibited surfaces with brittle cracks, indicating brittle fracture. IPN containing 80

wt% of epoxy and 20 wt% acrylate is characterized by a very smooth and ductile surface,

evidencing its elastomeric character (as explained in detail in the next section). The IPNs with

almost equivalent weight fractions of the two parent resins presented irregular and rough

surfaces, with a grooved, heterogeneous, and heavily striated morphology distributed throughout

the structure. As the proportion of this low molecular weight co-monomer increased, the brittle

character of the materials also increased, which is reflected in the highly glassy surface of the

100 wt% epoxidized α-resorcylic acid sample.

3.2.1.2 Thermo-mechanical characterization of IPNs

As mentioned earlier, the IPN system is composed of two different networks, the epoxy phase

and the acrylate phase. Table 4.1 shows the evolution of the storage modulus (E’) and glass

transition temperature as a function of epoxy/acrylate weight proportion.

Table 4.1. Evolution of the storage modulus (E’) and glass transition temperature as a function of

epoxy/acrylate weight proportion.

System Storage Modulus (MPa) at 30°C Glass Transition (°C)

Epoxidized linseed oil 100% 1286.80 68.1

IPN E80 – A20 318.58 39.8

IPN E60 – A40 6.87 27.8

IPN E40 – A60 13.95 25.0

IPN E20 – A80 363.53 44.8

Acrylated soybean oil 100 % 716.09 58.3

133

As seen in Table 4.1, compositional variations of epoxy/acrylate ratio deeply affected the

thermo-mechanical properties of the IPNs; and it is evident that the performance of the parent

systems (epoxy and acrylate networks) is considerably diminished when they are combined

forming IPNs. The storage modulus (E’) exhibited values of about 1.3 GPa and 0.7 GPa, and the

Tg was measured to be approx. 68°C and 58°C for the epoxy and acrylate systems, respectively,

indicating that materials behave as glassy polymers at ambient conditions. Since the parent resins

showed the highest thermo-mechanical performance, they can be easily visualized as the two

maximum reference points to correlated the thermo-mechanical properties of the IPNs. The

modulus of the IPNs varied as a function of their distance (in terms of composition) from the two

parent resins (the reference points) with values abruptly falling in between, reaching a minimum

for the IPN E60 – A40 and IPN E40 – A60. In the same manner, the Tg also considerably

decreased, reaching an average value of about 26°C for the IPNs with almost equivalent weight

fractions of the two parent resins. From the tan δ vs temperature plots, it was observed that there

was no phase separation, fact that was expected keeping in mind that the parent resins are both

triglyceride-derived materials.

The decline in the modulus values is directly correlated to the decrease in the density of active

chains or n; parameter that is often associated to the crosslinking density of the material (Figure

4.4). Conceptually, the higher the crosslinking density, the more constrained the chain motion;

and thus the stiffer and more thermally resistant the molecular structure is. Therefore, as the

crosslinking density increases, the hardness, modulus, mechanical strength, and chemical

resistance also increase [34].

134

Figure 4.4. Crosslinking density as a function of the epoxy weight fraction.

As seen in Figure 4.6, the value of n for the cured epoxidized linseed oil-SC 15 part B system is

approx. 1700 mol/m3 (epoxy weight fraction = 1). When acrylated soybean oil is incorporated

into the epoxy phase in a low proportion (20 wt.%), the n value severely falls about 12.5 times;

reaching a value of n = 137 mol/m3, typical of an elastomeric material. Common rubber bands,

for instance, have n = 190 mol/m3 [4]. As the acrylate weight fraction increases, the crosslinking

density also increases; which slightly improved their thermo-mechanical properties, as evidenced

by the rise of the Tg and modulus. However, the numerical values of the n parameter for the

IPNs still denoting the presence of poorly reticulated network, exhibiting long-range rubber

elasticity. When the weight fraction becomes 1 (100 wt% acrylate), the highest crosslinking

density of the series is reached (approx. 3200 mol/m3) indicating the behavior a brittle material

for this parent resin. Common thermosetting materials, such as commercial diglycidyl ether of

bisphenol A (DGEBA) type epoxy resin (EEW = 186 g/eq.) used in coating, adhesive, structural,

0

500

1000

1500

2000

2500

3000

3500

0 0.2 0.4 0.6 0.8 1

Cro

ssli

kin

g D

en

sity

, n

(m

ol/

m3)

Epoxy weight fraction

135

and composite applications shows an approximate value of n = 5000 mol/m3 when crosslinked

with a tetrafunctional amine [35].

Based on the thermo-mechanical characterization, it is evident that there is a plasticizing effect

of one system into the other as soon as they are combined to form an interpenetrated network;

the chains are being separated from each other and hence making crosslinking harder. Therefore,

with the aim of studying how the presence of one system impacts the polymerization of the

other, differential scanning calorimetry (DSC) was employed to evaluate the curing behavior of

the resins. When analyzing the behavior of the parent resins (epoxy and acrylate) it can be

clearly seen that there is a big gap in temperature, and therefore energy, between the onsets of

the polymerizations. The calorimetry indicated that the acrylated soybean oil-methyl

methacrylate system, containing highly reactive acrylate functionalities that chemically behave

as vinyl groups, begins to react at around 70°C.

This value coincides with the expected initial decomposition temperature of the AIBN, which at

this stage, is able to generate a small number of radicals that subsequently initiate the

polymerization by reacting with the acrylate functional groups of the triglyceride monomer. The

reaction proceeds up to a temperature of about 120°C, point at which the reaction seems to stop

due to the complete consumption of the monomers. On the other hand, the ring opening reaction

between the epoxy groups of linseed oil and the commercial amine SC-15 part B begins at

around 150°C, almost 90°C higher than its acrylate counterpart. It was also observed that the

offset of the curing peak, and therefore the complexion of the reaction, was at a temperature of

approximately 275°C.

136

It is well known that the radical chain polymerization consists of a sequence of three steps,

commonly referred to as initiation, propagation, and termination [25]. Once the acrylated

soybean oil-methyl methacrylate system reaches a temperature of about 70°C, the homolytic

dissociation of the AIBN (an azo compound) takes place; generating resonance-stabilized

cyanopropyl radicals that initiate the polymerization. Subsequent additions of a large number of

monomers fuel the propagation reaction, and finally at around 120°C; the propagating polymer

network stops growing and terminates. The situation seems to be quite different for the epoxy

system. During epoxy-amine curing process, the general reaction occurs via a nucleophilic attack

of the amine nitrogen on the carbon of the epoxy function. The mechanism proceeds through a

SN2 type reaction, and thus the reaction rate obeys second-order kinetics [36].

The direction in which an unsymmetrical epoxide ring is opened (or which epoxide carbon is

attacked by the nucleophile) depends on its structure and on the reaction conditions. If a basic

nucleophile is used in a typical SN2 reaction, such as an amine, the attack takes place at the less

hindered epoxide carbon. Although an ether oxygen is normally a poor leaving group in a SN2

reaction, the reactivity of the three-membered ring is sufficient to allow epoxides to react with

nucleophiles at elevated temperatures (approx. 100°C) [37]. However, the structural

characteristics of the monomers highly influence the reaction, and more aggressive conditions

might be needed to start the polymerization depending on the chemical environment of the

oxirane ring. Epoxidized oils (such as linseed oil) contain oxirane functionalities that are located

near the middle of the 18-carbon length fatty acid chains. As a consequence of this feature, these

groups are sterically hindered at both carbons, and also experience the effect of two electron-

injecting alkyl groups; decreasing their affinity for nucleophiles. Due to these two different

137

chemical effects, these epoxides react sluggishly with nucleophilic curing agents, such as amines

[38]; often requiring higher temperatures to overcome the energy barrier imposed by structural

characteristics in order to obtain the desired product. It is important to highlight that the

temperature require to achieve the complexion of the reaction needs to be counterbalanced with

the degradation temperature of the reactants and products.

Kowalski [39] studied the thermal-oxidative decomposition of oil and fats by performing DSC

scans of edible oils by heating the samples up to 360°C in an atmosphere of oxygen. Results

revealed that at around 160°C, the onset of the oxidative decomposition was observed. The final

stage of curing for the epoxidized linseed oil-acrylated soybean oil consisted of heating the resins

up to 180°C. In spite that the results of the DSC study in inert atmosphere suggested that

temperatures up to 275°C were needed to fully cure the resins, higher temperatures were not

tested, because after 1 hour at 180°C; signs of thermo-oxidative degradation were already evident

in the materials.

Based on these findings, it is suggested that one reaction begins first than the other, and that

there is a big gap between the onsets of the reactions. Initially, for rich acrylate soybean oil

systems, the acrylate monomers begun to react at around 70°C; leading to the formation of a

crosslinked network; whilst the epoxy monomers remained swollen in it. As the temperature

increased, it was expected that the formation of covalent bonds between the epoxy and amine

hardener took place. However, it seems to be that, due to limited molecular diffusion required for

polymerization (which fuels condensation reactions) [25], as well as to the very high

temperatures required to polymerize the epoxy system; the epoxy monomers remain unreacted or

138

partially reacted in the acrylic network, plasticizing it and therefore; leading to a decrease in the

thermo-mechanical properties of the IPNs. On the other hand, for rich epoxy systems, it is

evident that even in small proportions; the acrylated system severely reduces the ability of the

epoxy system to react, and hence making the crosslinking process very hard to accomplish.

From the obtained results, the acrylate soybean oil system seems to offer a good candidate to

formulate the interpenetrated networks, due to its ability to react at rather low temperatures and

the relatively good mechanical properties of the final product. However, the epoxidized linseed

oil system, as a result of its low reactivity caused by structural factors, does not stand as a

feasible option to develop IPNs systems. Since the goal of this project is to produce polymeric

materials of high thermo-mechanical performance, it was also decided to explore using highly

reactive phenolic-based epoxy resins to formulate the IPNs, keeping also in mind that the

thermal and mechanical performance properties of organic polymers has been generally linked to

the presence of rigid aromatic structures.

3.2.2 System B: epoxidized α-resorcylic acid/acrylated oil IPNs

3.2.2.1 Scanning Electron Microscopy

The fracture surfaces of the thermosets were studied by scanning electron microscopy (SEM).

The results of the morphological study are shown in Figure 4.5.

139

4.5.a

4.5.b

4.5.c

140

Figure 4.5. SEM micrographs of the fracture surface of 100 wt% epoxidized α-resorcylic acid

(4.5.a), IPN 75 wt% epoxy – 25 wt% acrylate (4.5.b), IPN 50 wt% epoxy – 50 wt% acrylate

(4.5.c), IPN 25 wt% epoxy – 75 wt% acrylate (4.5.d), 100 wt% acrylated soybean oil (4.5.e).

As shown, the fracture surfaces of the parent resins are characterized by smooth and

homogeneous microstructures; without any plastic deformation. Both the 100 wt% epoxy

resorcylic acid and the 100 wt% acrylated soybean oil formulations possess a fracture surface

4.5.d

4.5.e

141

and an internal structure of a brittle material, revealing a highly glassy surface; where crack

marks, ridges, and planes are dispersed all over the surface. The IPNs 75-25 wt% and 50-50 wt%

epoxy-acrylate are also characterized by a highly glassy surface. In the 25-75 wt%

epoxy/acrylate, the brittle character of the material seemed to decrease, and the surface exhibits

some degree of ductility and plastic deformation. As it will be discussed in the mechanical and

fracture studies, this sample showed the highest value for the critical energy release rate,

denoting its ability to dissipate energy due to its more ductile character. It was also evident from

the thermo-mechanical evaluation that this IPN presented the least brittle character of the series,

which was evident by its relatively high molecular weight between crosslinks. It is important to

highlight the morphology of the samples is consistent with the thermo-mechanical

characterization, which revealed a glass behavior at room conditions. Even though the starting

materials seemed to be heterogeneous in their chemical nature, there is no indication of phase

separation and no clear boundary between the phases. This indicated that the interpenetration

occurred at micro- or nano- size level [40], denoting that phase separation was kinetically

controlled by permanent interlocking of entangled chains.

3.2.2.2 Kinetics of gelation

The reaction between epoxy and amine groups is a step-growth reaction. The presence of three or

more epoxy groups leads to the formation of a tridimensional network when reacted with a

multifunctional amine. As a first approach to study the reactions, the gel times at 65°C were

measured (all measurement were performed three times). In order to study how the presence of

one system influences the crosslinking behavior of the other, a hypothetical epoxy system (in

which the acrylate phase has no initiator), and a hypothetical acrylate system (in which the epoxy

142

phase has no curing agent); were prepared and measured, along with the experimental system

(containing all the components to allow the polymerization of the monomers). The resulting gel

times, as a function of the weight fraction, are shown in Figure 4.6.

Figure 4.6. Gel times at 65°C for the studied IPN systems based on epoxidized α-resorcylic acid

and acrylated soybean oil.

As can be seen, all the systems reached the gel point in less than approx. 60 minutes when tested

at 65°C; and this parameter changed as a function of the epoxy/acrylate ratio. It was observed

that the gel time of the hypothetical systems increased when increasing the content of the other

phase. In other words, the gel point of the acrylate phase was delayed when the concentration of

the epoxy phase in the mixture was enlarged, and vice versa. Based on these finding, it is

suggested that there is a dilution of the reactants by the components of the other non-reacting

phase, decreasing the reaction rate and therefore; inducing the delay in the gelling time.

Regarding the real system, in which all the reacting components were included, it is interesting

to observe that for the epoxy - acrylate ratios of 25 wt% -75 wt% and 50 wt% - 50 wt%, the gel

times were considerably reduced, when compared to those shown by the hypothetical systems.

Hypothetical

Epoxy System

(red)

Hypothetical

Acrylate System

(blue)

Experimental

System (green)

0

10

20

30

40

50

60

70

0 0.25 0.5 0.75 1

Ge

l ti

me

(m

inu

tes)

at

65

°C

Epoxy phase composition (weight fraction)

143

The other ratios showed gelling times similar to the hypothetical counterparts. The system Epoxy

75 wt% - Acrylate 25 wt% reached the gel point at a similar time (~ 27.5 minutes), to that shown

by the hypothetical epoxy system (no initiator in the acrylate phase); denoting that the gelling of

this system is dominated by the epoxy phase.

As demonstrated by the DSC studies (detailed in the following section), the condensation

reaction of the epoxy monomers with the crosslinking agent (amine in this case) is highly

exothermic. It is important to mention that this is an expected behavior, because the ring opening

reaction of the highly strained three-member ring occurs easily, and is thermodynamically

favorable. On the other hand, it has been clearly stated by theory that the radical chain

polymerization rate is dependent on the initiator concentration [25]. Usually, thermal homolytic

dissociation of initiators is the most widely used method of generating radicals to start the

polymerization. Taking these facts into account, it is likely that the decrease in the gelling time

for the epoxy - acrylate ratios of 25 wt% - 75 wt% and 50 wt% - 50 wt% is due to a phenomenon

of auto-catalysis. The ring opening reaction of the highly strained three-member epoxy ring

releases extra energy in the system, favoring the dissociation of the initiator and therefore,

increasing the polymerization rate that leads to gelation at some point of the reaction. This

behavior, combined with the gelling of the epoxy phase, might have maximized the rate at which

the systems reached the gel point.

The theoretical extent of the reaction at the gel point was calculated by means of the Carothers

and Flory-Stockmayer approaches. According to Carothers, Equation (4.3) yields an average

functionality of 4.01 for an equimolar α-resorcylic acid/Jeffamine T-403 mix. Equation (4.2)

144

then yields Pc = 0.499. According to Flory and Stockmayer, taking fw,epoxy = 3, and fw,amine = 6,

equation (4.4) yields Pc = 0.316. The extent of the reaction, at 65°C, was followed by studying

the disappearance of the oxirane absorption band at 825 cm-1 (the measurements were performed

three times). The observed Pc value for a stoichiometric mixture of epoxidized α-resorcylic acid

and multifunctional amine was 0.430 ± 0.05 (Figure 4.7). Experimental values falling

approximately midway between the two calculate values have been observed in many other

similar systems [25]. The extent of the reaction at the gel point for the remaining IPNs is shown

in Table 4.2.

Figure 4.7. Extent of conversion of stoichiometric mixture of epoxidized α-resorcylic acid and

multifunctional amine (Jeffamine T-403)

Epoxy phase

100%

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Ex

ten

t o

f co

mv

ers

ion

(%

)

Time (min)

Gel Time

145

Table 4.2. Extent of the reaction at the gel point for IPNs based on epoxidized α-resorcylic acid

and acrylated soybean oil.

IPN Epoxy phase conversion Acrylate phase conversion

Epoxy 100% 0.43 -------------------

Epoxy 75% - Acrylate 25% 0.40 0.47

Epoxy 50% - Acrylate 50% 0.55 0.43

Epoxy 25% - Acrylate 75% 0.50 0.20

Acrylate 100% ----------------------- 0.21

As can be seen, the conversion of the epoxy phase at the gel point was not deeply affected by the

presence of the acrylate phase, and it showed a medium conversion, when tested at 65°C. This

parameter ranged from 40 % to 55 %, and there was no clear trend in its behavior as a function

of the ratio epoxy/acrylate. On the other hand, the acrylate phase conversion was impacted by the

presence of the epoxy phase. Samples containing 100 wt% and 75 wt% of acrylate reached the

gel point at similar conversions (approx. 20 %), but at different times, with the Acrylate 75 wt%

system reaching the gelation almost two times faster than the Acrylate 100 wt%. When

increasing the weight content of the epoxy phase in the IPNs, the conversion of the acrylate at

the gel point also increased, reaching its maximum (approx. 96 %) for the system Epoxy 75 wt%

- Acrylate 25 wt%. The rise in the conversion values at the gel point for the IPNs Epoxy 50 wt%

- Acrylate 50 wt% and Epoxy 75 wt% - Acrylate 25 wt%, could be attributed to an autocatalysis

phenomenon in the system; due to the energy release of the ring opening reaction of the epoxy

ring when combined with the amine curing agent, as previously explained. As a general trend,

for large epoxy contents, the heat released during reaction might have triggered the conversion of

146

the acrylate phase; however, since its content is low, the gelling is dominated by the epoxy

counterpart.

3.2.2.3 Curing behavior of IPNs

In order to assess the curing of the epoxy – acrylate IPNs, about 5 mg of each mix was studied

by DSC (all measurements were performed three times). Likewise, the samples were cured at

65°C for 1 hour and then also subjected to calorimetric analysis to evaluate the extension of

curing after treatment at the above mentioned conditions. Table 4.3 shows the maximum values

of the curing peak temperatures for the IPNs, as well as, the onset of the peaks

Table 4.3. Onset and maximum of curing peak temperatures for the IPNs based on epoxidized α-

resorcylic acid and acrylated soybean oil.

System Onset of

Curing Peak

(°C)

Maximum of

Curing Peak

(°C)

Maximum of Curing Peak (°C)

After curing 1 hr. at 65 °C

Epoxy 100 wt% 65.82 102.66 89.89

Epoxy 75 wt% - Acrylate 25 wt% 68.80 103.65 103.53

Epoxy 50 wt% - Acrylate 50 wt% 73.98 100.43 97.79

Epoxy 25 wt% - Acrylate 75 wt% 74.99 93.40 115.14

Acrylate 100 wt% 76.62 86.42 116.61

The DSC of the IPNs, as well as the pure systems, showed only one exothermic peak,

corresponding to the curing of the resins. From Table 4.3, it can be observed that the onset of the

curing is shifted to higher temperatures when increasing the amount of acrylate resin in the

147

mixture. When analyzing the behavior of the parent resins (epoxy and acrylate) it can be

inferred that the polymerization of both systems begins at close temperatures, with the epoxy

system starting to react at a temperature around 10°C lower than the acrylate resin. As a result,

there is only one peak observed in the IPNs, which corresponds to the superimposed curing of

both resins. From the calorimetry studies, it was also observed that the offset of the curing peaks

was around 160°C. The total heat of polymerization and the residual heat of polymerization

(residual heat after curing 1 hour at 65°C) are displayed in Figure 4.8.

0

10

20

30

40

50

60

70

80

0 0.25 0.5 0.75 1

ΔH

(J/

g)

Epoxy Weight Content

Total Curing Heat Residual Curing heat 4.8.a

148

Figure 4.8. Heats of polymerization (4.8.a) and conversions (4.8.b) of the IPNs based on

epoxidized α-resorcylic acid and acrylated soybean oil.

When analyzing the behavior of the parent resins, it is clear that the ΔH of curing of the epoxy-

amine resin is highly exothermic (approx. 67 J/g), being about 2.5 times higher than that shown

by the acrylate phase (26 J/g). This means that mixtures richer in the epoxy phase are more

exothermic, behavior that is reflected in the experimental results, where the overall or total heat

of polymerization associated with the curing peaks is shifted to higher values when increasing

the content of epoxy phase in the systems. The total heat of polymerization approximately

increases as a linear function of the epoxy content; however this tendency is truncated by the

Epoxy 25 wt% - Acrylate 75 wt%, suggesting that the curing mechanism of each constituent is

affected by the presence of the other, probably due to autocatalysis or even chemical interactions

between the components [1].

75

77

79

81

83

85

87

89

91

93

95

0 0.25 0.5 0.75 1

Co

nv

ers

ion

(%

)

Epoxy Weight Fraction

4.8.b

149

Regarding the residual heat after curing 1 hour at 65°C, it seems to be not greatly affected by the

ratio epoxy-acrylate, and it ranged from around 4.000 J/g to 8.500 J/g. This finding suggests that

most of the components are reacted after 1 hour at 65°C, fact that is reflected in the conversions

(Figure 8); which ranged from around 78 % to 91 %. As happened with the curing heats, the

conversions shifted to higher values when increasing the proportion of the epoxy system;

probably due to the high chemical reactivity of non-sterically hindered glycidyl ethers, such as

epoxidized α-resorcylic acid, towards amines [41].

Concerning this reaction, the two amino hydrogens in primary amines, such as Jeffamine T-403,

have initially the same reactivity but once the first one has reacted, the secondary amine formed

may be less reactive [34]. As previously described, during epoxy-amine curing process, the

general reaction occurs via a nucleophilic attack of the amine nitrogen on the carbon of the

epoxy function. The mechanism proceeds through a SN2 type reaction, which is in turn highly

dependent on the structure and on the reaction conditions. Epoxidized α-resorcylic acid contains

oxirane functionalities that are non-sterically hindered, facilitating the nucleophilic attack of the

amino group. As a consequence of this feature, the reaction occurs rapidly at relatively low

temperatures (~ 65°C). It is important to mention that etherification products, arising from the

polymerization of epoxy groups and hydroxyl groups of acrylated soybean oil at elevated

temperatures, might be also present in the final polymer matrix. However, since stoichiometric

amounts of epoxy groups and amine comonomers are used, and also due to the low reactivity of

the secondary hydroxyl groups [42]; the effect of this side reaction on the final product can be

neglected.

150

Based on the results of the conversion and calorimetry studies, it is suggested that the two non-

competing reactions (step and free radical polymerizations) occurred at very close temperatures,

with a very small gap of around 10°C between the onsets of the reactions. However, it is clear

that each constituent is affected by the presence of the other, altering at rate and extent at which

each individual reaction takes place.

3.2.2.4 Thermo-mechanical characterization.

Figure 4.11 shows the dynamic mechanical response of IPNs based on epoxidized α-resorcylic

acid and acrylated soybean oil. The finding of a well-defined transition from the glassy state to

the rubbery state, follow by the rubbery plateau is consistent with the dynamic mechanical

behavior of other thermosetting polymers, indicating the existence of a stable network structure

(4.9.a)

Epoxy 100%

Epox. 75% -

Acry. 25%

Epox. 50% -

Acry. 50%

Epox. 25% -

Acry. 75%

Acrylate 100%

1.E+07

1.E+08

1.E+09

1.E+10

20 40 60 80 100 120 140 160 180 200

Sto

rag

e M

od

ulu

s (P

a)

Temperature (°C)

4.9.a

151

Figure 4.9. Dynamic mechanical response of IPNs based on epoxidized α-resorcylic acid and

acrylated soybean oil. Storage modulus (4.9.a) and loss factor or tan δ (4.9.b) as a function of the

composition of the IPNs

In the glassy region, the systems showed a high modulus (~ 0.7 – 3 GPa). It is clearly seen that

with increasing the acrylated soybean oil content, the stiffness (E`) in the glassy state is

considerably decreased. Once the glass transition was surpassed, all of the resins exhibited a

rubbery plateau modulus that was insensitive to the temperature (~ 16 – 65 MPa). The plateau

moduli of the IPNs also decreased with increasing the acrylated oil content, which reduces the

crosslinking density or n values, as established by the proportionality between E` and this

parameter (Table 4.4). From the plots, it is evident that the epoxy parent system is more

crosslinked that the acrylate parent system, and that the crosslinking is also severely reduced

when the acrylate soybean oil content is increased in the systems. The crosslinking effect on Tg,

is an increasing function of the chain stiffness, which is under the dependence of molecular scale

factors, essentially aromaticity. The chain stiffness expressed by the flex parameter F which, for

Epoxy 100

Epox. 75% -

Acry. 25%

Epox. 50% -

Acry. 50%

Epox. 25% -

Acry. 75%Acrylate

100%

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

25 50 75 100 125 150 175 200

tan

δ

Temperature (°C)

4.9.b

152

a given chain of molar mass Me, is given by F = Me/Ne, where Ne is the number of elementary

(undeformable) segments. F is essentially an increase function of the content of aromatic nuclei

[43]. The α-resorcylic acid based epoxy matrix is structurally formed by aromatic units,

polyfunctional hard clusters that constitute a rigid and densely packed phase that largely reduces

the coordinated large scale motions in the network. It is therefore evident that mixtures richer in

the epoxy phase are stiffer, conferring support to the network when stress is applied.

The reduction in the stiffness of the systems is also accompanied by a similar change in the Tg,

as evidenced by the shift in the α-transition temperature peak from high to low temperatures.

The peaks of the parent resins were relatively narrow, whilst the IPNs (due to the multitude of

components and the great amount of possible relaxations modes acting in triglyceride-based

polymers) showed a wider transition [44]. Likewise, all the systems showed only one tan δ peak,

indicating that there was no phase separation and therefore, good compatibility between the

epoxy and acrylate phase. It is important to mention that there seems to be a decrease in the

intensity of the Tg peaks in the tan delta plots of the IPNs. This intensity reduction of the Tg

peak can be ascribed to some constraint effects on supramolecular level which supports the

presence of interpenetrating of the two networks [45]. Note that a reduction in Tg is usually due

to less tight crosslinking (co-network) which covers also the formation of IPN. Moreover, the

peak width at half height is a criterion used to indicate the homogeneity of the amorphous phase.

A higher value implies higher inhomogeneity of the amorphous phase. In this particular case, the

widening of the loss factor peak was observed for the IPNs, especially for the 50 wt% - 50 wt%

mixture. This behavior could be associated to less cross-linked and heterogeneous networks [46].

153

The resulting thermo-mechanical properties of the systems are listed in Table 4.4 The

crosslinking density (n) and molecular weight between crosslinks (Mc) were estimated from

experimental data using the rubber elasticity theory (Equation 4.5) to be able to explain the

effects of the phases on the mechanical properties of the resulting IPNs.

Table 4.4 Mechanical Properties of the IPNs based on epoxidized α-resorcylic acid and acrylated

soybean oil.

System

Tg

(°C)

E` at 30°C

(GPa)

n

(g/cm3)

n

(mol/m3)

Mc

(g/mol)

Epoxy 100% 129.10 3.28 1.1301 ± 0.0315 6026 188

E75% - A25% 104.18 2.43 1.1293 ± 0.0488 3431 329

E50% - A50% 89.11 1.56 1.1169 ± 0.0272 2235 500

E25% - A75% 56.41 0.55 1.0984 ± 0.0019 1872 587

Acrylate 100% 58.27 0.72 1.0815 ± 0.0084 3185 340

For the calculation of n, it was assumed that all of the components in the systems entered into the

network, which is in reality impossible. However, after the Soxhlet extraction of samples with

acetone for 6 hours, it was observed that the total amount of monomers that did not polymerize

ranged from 3 % to 7% (from Epoxy 100% to Acrylate 100%). These results allowed assuming

that the influence of low incorporation of monomers into the networks can be neglected.

As seen in the table, all of the systems behaved as brittle glassy polymers at room conditions.

The value of n for the epoxidized α-resorcylic acid cured with Jeffamine T-403 system is approx.

6026 mol/m3, denoting a highly crosslinked network (similar to a diglycidyl ether of bisphenol A

(DGEBA) type epoxy resin, having n = 5000 mol/m3). When acrylated soybean oil is

154

incorporated into the epoxy phase in a low proportion (25 wt.%), the n value fell about 2 times;

reaching a value of n = 3431 mol/m3, As the acrylate weight fraction increases, the crosslinking

density still decreasing; lowering the thermo-mechanical properties, as evidenced by the

numerical decrease of the Tg and modulus. For the 100 wt% acrylate soybean oil system, the

crosslinking density reached approx. 3185 mol/m3. It is clear from these results that the acrylated

soybean oil phase is plasticizing the epoxy network. The dynamic mechanic behavior previously

described comprises a balance between two factors, crosslinking density and plasticization. As

the crosslinking density increases the more constrained the chain motion; and thus the stiffer and

more thermally resistant the molecular structure is. Plasticization is due to the molecular nature

of the triglyceride. The positional distribution of the acrylate groups imply that dangling chain

ends remains after crosslinking of the acrylate moieties. Also, the soybean oil contains fatty acids

that are completely saturated and cannot be functionalized with acrylates. These inelastically

active fatty acid dangling chains do not support stress when a load is applied; therefore they act

as a plasticizing agent, introducing free volume and enabling the network to deform easily when

stress is applied to the material. This plasticizer effect is inherent to all natural triglyceride-based

materials; and it represents an issue for their potential use in high performance applications [47].

Likewise, based on property modeling of triglyceride-based thermosetting resins, La Scala and

coworkers [48] have also suggested that side reactions during crosslinking of fatty acids might

take place. Intramolecular cyclization of functional groups on the same triglyceride reduces the

number of effective attachment points of this triglyceride to the rest of the network. When a

triglyceride reacts with itself before reacting with another triglyceride, it forms a loop that is not

attached to the rest of the polymer network. Consequently, this reduces the crosslink density of

155

the resulting polymer. Every time two functional groups react in this way, the effective number

of functional groups per triglyceride is reduced by 2. As revealed by the results, the polymer

densities increased slightly with the increase in the epoxy phase content, and ranged from about

1.08 to 1.13 g/cm3. Similar values for triglyceride based materials have been reported by La

Scala and Wool [49]. The rubbery moduli of the systems ranged from approx. 16 to 65 MPa,

yielding molecular weight between crosslinks ranging from approx. 188 to 588 g/mol. The Mc,

as expected, increased with the increase in the acrylated soybean oil content, because the fatty

acid chains increase the free volume of the polymer matrix. Similar values have been obtained by

[44,50].

As a summary of the thermos-mechanical results, α-resorcylic acid based epoxy system provided

a considerable better option to formulate the interpenetrated networks, overcoming the

drawbacks given by its triglyceride-based epoxy counterpart, due to its ability to react at rather

low temperatures and the very good mechanical properties. However, no synergistic effect on the

thermo-mechanical properties was observed, and contrary to this a reduction in the mechanical

performance was observed in the IPN, when compared to their parent resins; mainly attributable

to the plasticizer effect conferred by the triglyceride-based acrylate matrix.

3.2.2.5 Linear Elastic Fracture Mechanics

In order to investigate the influence on the introduction of the acrylate phase on the epoxy phase,

and vice versa, the fracture toughness of the IPNs and parent resins was evaluated through the

critical stress intensity factor, KIc. Table 4.5 depicts the obtained results.

156

Table 4.5. Experimental KIc and GIc values

System KIc (MPa·m1/2

) GIc (kJ/m2)

Epoxy 100% 1.803 ± 0.238 0.88

E75% - A25% 1.593 ± 0.245 0.93

E50% - A50% 2.100 ± 0.027 2.52

E25% - A75% 1.652 ± 0.159 4.45

Acrylate 100% 0.842 ± 0.086 0.88

According to the statistical analysis of the results, there is significant variation in the fracture

toughness of the IPNs when compared to their parent resins (P < 0.05). However, Tukey`s test

indicated that the data can be classified into three groups of observations, having means that are

significally different. There is statistical evidence to support that the epoxy 50 wt% - acrylate 50

wt% IPN showed the highest fracture toughness of the series (first group of observations),

exhibiting a KIc value of around 2.1 MPa·m1/2. The secong group of observations comprises the

100 wt% acrylate, which showed the poorest resistance to fracture (around 0.84 MPa·m1/2). The

third group emcompasses all of the remaining systems, in which there is no evidence to support

that there`s a significant difference in the value of this property, meaning that the critical stress

intensity factor is not changing as a function of the epoxy/acrylate ratio. The result clearly

showed that the inherent fracture toughness of the IPNs is only a weak function of the

epoxy/acrylate weight ratio. However, it is important to highlight that the toughenability of the

systems depends on the crossliking density of the matrix. The lower the crossliking density, the

greater the thoughenability. Recalling from Table 4, the IPNs epoxy 50 wt% - acrylate 50 wt%

and the epoxy 25 wt% - 75 wt%, showed values of n equal to 2225 and 1872 mol/m3,

respectively. These systems showed the lowest crosslinking density, and therefore exhibited

greater ductibility and correspondenly, greater toughenability. The effect of reducing crossliking

157

density has been proposed to be an increase in the mainchain mobility which results in increased

ductility [51]. It is well known that plastic deformation is localized at the crack tip during crack

propagation in highly crosslinked thermosetting resins, and large plastic deformation occurs if

crossliking is light. Therefore, the increase in the fracture toughness with the decrease in the

croslinking density is mainly attributed to plastic deformation arising from easy chain flow [52].

For commercial diglycidyl ether of bisphenol A (DGEBA) type epoxy resin, crosslinked with

Jeffamine T-403, KIc was found to be approx. 0.80 MPa·m1/2 [53]. Regarding commercial

polymethylacrylate (PMMA), previous investigations performed by this group indicated a value

of around 1.1 MPa·m1/2 [5].

The critical energy release rate, GIc, can be related to the critical stress-intensity favor, KIc, in

plane strain conditions by the Equation (4.8). The storage modulus is a measure of the energy

stored elastically during deformation, and ideally is equivalent to Young`s modulus. However,

this is not true in real conditions, and their values might not be equal, mainly because the tests

are very different. The Young`s modulus is calculated from a stress-strain test, in which the

material is constantly stretched; whereas for the storage modulus, the material is oscillated in

dynamic test [4]. However, in spite this difference, storage modulus was used as an

approximation to Young`s modulus (E` ~ E) to establish some numerical values for the fracture

energy. As previously stated, in glassy polymers most of the energy necessary to propagate a

crack is due to the plastic deformation near the crack tip. This energy in macroscopically

measurable through the critical energy release rate, and the actual failure of the material require

molecular events such as chain pullout or chain scission within, or adjacent to, the plastic zone.

As seen in the table, the parent resins showed equal fracture energies, having values of GIc = 0.88

158

kJ/m2. Unlike the fracture toughness; the fracture energy showed a dependence on the

epoxy/acrylate ratio, increasing as the acrylate proportion is enlarged. The highest values were

exhibited by the IPNs epoxy 50 wt% - acrylate 50 wt% and the epoxy 25 wt% - 75 wt%, with GIc

= 2.5 kJ/m2 and GIc = 4.5 kJ/m2, respectively. The variation of GIc as a function of this relation

reflects both the increase in the toughness, due to its direct proportionality with KIc, and the

decrease in the storage modulus. The introduction of the acrylate phase into the epoxy phase

decreased the crosslinking density, favoring the chain flexibility as a consequence of having

higher Mc values. It is accepted that thermosetting resins undergo plastic deformation via shear

yielding rather than crazing, and this is attributed to high crosslinking density and small chain

contour length, le. However, when crosslinking decreases the chain contour length grows; the

materials become more ductile and tend to yield, dissipating energy through chain deformations

and molecular motion [52]. According to Karger-Kocsis and Gremmels (2000) [55], the fracture

toughness and energy toughness can be described by the rubber elasticity theory. It is established

that under plane strain conditions, KIc and GIc change as a function of the molecular weight

between crosslinks by linear and square root relations, respectively. Both relations were verified.

It was found that there is no a linear correlation between KIc and Mc for the systems under study.

The results showed that the KIc values did not increase as Mc increased, which seems to suggest

that a different mechanism is responsible for the increase in the fracture toughness displayed by

IPNs. It is clear however, that the high toughenability was correlated to high molecular weight

between crosslinks.

On the other hand, a similar analog was studied for the relation between the fracture energy and

Mc. Figure 4.10 depicts the results.

159

Figure 4.10. GIc versus Mc1/2 for the studied IPNs

Literature suggests that GIc generally increases with Mc [31], and some of the obtained results are

in agreement with the previous relation. As seen, there seems to be a linear relation between the

energy and the square root of Mc for the IPNs. In this case, the introduction of the flexible

acrylate soybean oil phase increases the molecular weight between crosslinks, and therefore

increases the fracture energy. This behavior is directly linked to the influence of the flexibility of

the chains between crosslinks, which improves the toughness due to the fact that the longer the

chains the more flexible they become. However, it is important to mention that the linear

dependency does not hold for the entire set of values. There is no correlation between the IPNs

and the parent resins, whose fracture behavior seems to be governed by a different mechanism.

Epoxy 100%

Epoxt 75 % -

Acryl. 25 %

Epox. 50 % -

Acryl. 50 %

Epox. 25 % -

Acryl. 75 %

Acrylate 100 %

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

10 12 14 16 18 20 22 24 26

GIc

(kJ/

m2

)

Mc1/2 [(g/mol)]1/2

160

4. Conclusions

IPNs have been prepared using an epoxy phase synthesized from linseed oil and/or α-resorcylic

acid and acrylate phase using acrylated soybean oil. The E.E.W. of the epoxy monomers after

chemical functionalization was found to be approx. 171 and 112 g/eq. for the epoxidized

derivatives of α-resorcylic acid and linseed oil, respectively. Grafting of epoxy functions onto

the monomer`s structure was confirmed by FTIR. DSC analysis revealed the presence of

exothermic peaks related to curing of the systems, and that the curing mechanism of each

constituent was affected by the presence of the other. The experimental Pc value for a

stoichiometric mixture of epoxidized α-resorcylic acid and multifunctional amine was 0.430 ±

0.100; falling in between the predicted by the Carothers and Flory theories. The mechanical

properties of the IPNs were not superior to those of the constituent polymers as a result of the

plasticizing effect of the triglyceride flexible chains in the macromolecular assembly of the IPNs.

However, IPNs based on epoxidized α-resorcylic acid and acrylated soy bean oil provided

considerably better mechanical properties than the system epoxidized linseed oil and acrylated

soy bean oil. For the α-resorcylic acid and soy bean oil system, modulus values ranged from 0.7

to 3.3 GPa at 30°C, and glass transition temperatures ranging from around 58 °C to approx.

130°C. Both parameters were affected by the epoxy/acrylate ratio, showing an increase as the

epoxy proportion was enlarged. Interpenetrating networks containing equivalent weight

proportions of the parent resins showed the highest fracture toughness of the series, exhibiting a

KIc value of around 2.1 MPa·m1/2. Likewise, FTIR and Soxhlet extractions indicated high degree

of conversion and incorporation of the monomers into the polymer networks. Based on the

results of this study, it is clear that the triglyceride-derived IPNs behave as elastomers at room

conditions; meanwhile the IPN in which modified α-resorcylic acid was introduced as the epoxy

161

phase correspond to glassy polymers at ambient conditions. The latter could be used as

thermosetting matrices for the development of composite materials. Likewise, they have

potential as partial replacements in the formulation of commercial thermosetting resins where

higher toughenability than their parent epoxy and acrylate resins is requiered.

162

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169

CHAPTER V

Fast Pyrolysis Bio-oil as Precursor of Thermosetting Epoxy Resins

1. Introduction

The use of biomass-based resources is currently a hot topic for both academic and industrial

research as an alternative to mitigate the strong dependence on fossil carbon in the chemical and

petrochemical industries. Lignocellulosic materials, being abundant and renewable (with annual

production arising to more than 1 billion dry tons in the U.S.A) [1], are the most favored

substrate for bioconversion. It is clear that the development of green polymers and materials is

growing, carrying along a vast and promising area for scientific research; that is supported not

just by the industrial or academic sectors, but also by governmental entities and agencies.

According to Effendi et al. [2], lignocellulosic biomass is made up of three main components:

hemicellulose, cellulose and lignin, of which the lignin fraction can account for up to 40% of its

dry weight. Lignin is an amorphous cross-linked polymer with very complicated structure. It can

be described as a three-dimensional, highly branched, polyphenolic substance consisting of an

irregular array of variously bonded “hydroxyl-“ and “methoxy”-substituted phenyl propane units.

These monomeric phenylpropane units exhibit p-coumaryl, coniferyl, and sinapsyl structures,

which are shown in Figure 5.1 [3].

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Figure 5.1. Monomeric phenylpropane unit structures.

Source: D. Mohan, C.U. Pittman, P.H. Steele, Energy & Fuels. 20 (2006)

Lignin based materials can be obtained in very large quantities as by-products of the pulping

industry. These products are, as a matter of fact, fragments of the natural polymer; resulting from

different chemical splicing mechanisms during processing [4]. The exploitation of lignin as a

source of profitable chemicals or macromonomers for the polymer industry is not just backed up

by the huge amounts of this material available after the traditional pulping processes; but also by

the high content of aromatic moieties, containing aliphatic and phenolic hydroxyl groups

(present in different frequencies and proportions), which are of particular interest because they

serve as key monomers to synthesize high performance polymers [5].

The idea of using unmodified lignin for the preparation of epoxy resins was firstly reported in the

late 1990s; in which Kraft lignins were combined with different epoxy monomers and then

crosslinked with commercial aromatic diamines [6]. On the other hand, with the aim of obtaining

a modified lignin with enhanced reactivity (due to poor results with unmodified lignins), Glasser

et al. [6,7] reported the synthesis of lignin based epoxy resins by reacting partially oxypropylated

lignins (previously obtained by treating lignin with diethyl sulfate and then with propylene oxide

OH

OH

OH

OH

O

CH3

OH

OH

O O

CH3

CH3

p-Coumaryl alcohol Coniferyl alcohol Sinapyl alcohol

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to “bring out” the -OH groups, and thus make them more available to react) with

epichlorohydrin. After modification, the lignin based material bearing epoxy moieties was also

reacted with an aromatic diamine. Even though studies like this concerning the use of lignin as a

component for macromolecular materials have been reported and published [8,9], often synthesis

strategies addressed to modify this material struggle with low reactivity associated to steric

hindrance; making difficult to insure good access to the targeted functional groups [4].

Some novel technologies based on biomass refinery appear as a good alternative to overcome

this problem, and stand as a sustainable way for aromatics biosourcing, by providing low

molecular weight fragments of natural lignin, whose functional groups are more exposed to the

reaction medium (due to their lesser steric crowding), and thus being readily reachable during

chemical synthesis. The thermochemical conversion of lignocellulosic biomass (i.e., combustion,

pyrolysis, or gasification) is now receiving an increasing interest as a method for production of

renewable energy, fuels, and high added value chemicals. When heated, the lignin component

depolymerizes to form monomeric and oligomeric phenolic compounds. It terminally

decomposes over a wide temperature range (approx. 160-900°C), and a possible free radical

mechanism for its decomposition has been proposed for several authors. Amongst them, Evans

et al. [10] suggested that alkyl hydroxyl groups in the α-position are first eliminated from the

propane side chain, followed by β-ether cleavage. Further details about the mechanism have been

recently described by Shen and others [11,12].

Amongst several thermal degradation processes, fast pyrolysis (also known as thermolysis) has

been developed relatively recent. Fast pyrolysis is a high-temperature process in which biomass

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is rapidly heated in the absence of oxygen. Biomass decomposes to generate vapors, aerosols,

and some charcoal-like char. After cooling and condensation of the vapors and aerosols, a dark

brown viscous liquid is formed. This process produces 60-75 wt% of liquid bio-oil, 15- 25 wt%

of solid char, and 10-20 wt% of non-condensable gases (depending on the feedstock used) [3].

Balat et al. [13] has recognized four essential features of a fast pyrolysis process: (a) very high

heating and heat transfer rates are used, usually requiring a finely ground biomass feed, (b)

careful control of the pyrolysis temperature, commonly ranging from 700 to 770 K, (c) short

vapor residence times, typically less than 2 seconds, and (d) pyrolysis vapors and aerosols are

rapidly cooled to give bio-oil. These oils are usually dark brown colored multicomponent

mixtures comprised of hundreds of organic oxygen-containing components of different size

molecules, derived primarily from de-polymerization and fragmentation reactions of the main

biomass components [14]. Some of the major components found in pyrolysis oil are shown in

Table 5.1.

Table 5.1. Major components found in pyrolysis oil

Major components Abundance (wt%)

Aldehydes 10-20

Alcohols 2-5

Carboxylic acids 4-15

Furans 1-4

Ketones 1-5

Phenolic Monomers 2-5

Phenolic Oligomers 15-30

Sugars 20-35

Water 20-30 Source: M. Staš, D. Kubička, J. Chudoba, M. Pospíšil, Energy and Fuels. 28 (2014) 385–402.

174

Due to the presence of this vast amount of different and multifunctional compounds, the crude

bio-oil could serve as raw materials for the obtention and production (after further processing

and separation) of specialty chemicals and resins. As a renewable liquid fuel, bio-oil can be

easily stored and transported; however, compared to heavy petroleum fuel oil, bio-oils have

certain properties limiting their application as fuels. Some of these include high water content,

high viscosity, high ash content, high corrosiveness (acidity) and high oxygen content (low

heating value) [15]. Regarding this last feature, bio-oils contain up to 45% oxygen, and this

element is a component of most of the more than 400 compounds that have been identified in

pyrolysis oils. Phenolic compounds found in bio-oils are composed of both simple phenols and

oligomeric polyphenols; having multiple phenol structural units and containing varying number

of acidic, phenolic and carboxylic acid hydroxyl groups, as well as aldehyde, alcohol, and ether

functions [12]. The molecular weight of these oligomers (product of lignin decomposition)

ranges from several hundreds to 5000 g/mol, depending on the pyrolysis process [2].

Many commercial epoxy monomers usually have an aromatic structure, conferring thermo-

mechanical stability to the network. As seen in Chapters III and IV, the inclusion of α-resorcylic

acid considerable improved the mechanical properties of the resulting resins. It is clear that there

is a great potential to develop high-performance resins, and the problem of finding a renewable

source of aromatic materials needs to be addressed. Thus, in this context, bio-oil phenolic

derivatives are excellent candidates for the bio-sourcing of aromatic monomers that can be used

to synthesize epoxy resins and other resins. These compound present potential as fully or partial

replacements in the formulation of commercial epoxy resins used as coatings, adhesives or even

for the preparation of composite materials. Therefore, the aim of this work was to synthetize a

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high performance bio-based epoxy polymer using fast pyrolysis bio-oil (containing lignin

fragments) as a source of phenolic compounds. The strategy to prepare bio-oil based polymers

can be divided into two sections. The first one comprises the chemical characterization of bio-

oils using chromatographic and spectroscopic methods, focusing on performing a qualitative

identification of the interest compounds, as well as a quantitative analysis of the targeted

functional groups. In a second step, the modified monomers are polymerized and the resulting

materials characterized, mainly focusing on their thermos-mechanical behavior. Likewise, α-

resorcylic acid (a phenolic acid with two hydroxyl and one carboxylic acid functional groups)

was also modified with reactive epoxy moieties, and used as low molecular weight comonomer

to enhance the density of aromatic structures in the crosslinked epoxy network.

2.Experimental section

2.1 Materials

Fast pyrolysis bio-oil, from hardwood (Oak), was supplied by the Department of Biosystems

Engineering, Auburn University, Alabama, United States. α-resorcylic acid or 3,5-

dihydroxybenzoic acid (α-RA, ≥ 98%), epichlorohydrin (99.0 %), and sodium hydroxide (NaOH,

≥ 98%), were purchased from VWR (US). Reagent grade chloroform, pyridine, and

dichloromethane were also purchased from VWR (US). Benzyltriethylammonium chloride

(BnEt3NCl, ≥ 98%), used as a phase transfer catalyst, was also acquired from VWR (US). 2-

chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP, 95.0 %) was used as

phosphorylating agent. 4-dimethylaminopyridine (DMAP, 99.0 %) was used as initiator for the

chain homopolymerization of epoxy monomers. Super Sap 100/1000 epoxy resin/hardener, by

Entropy Resins, with a bio-based carbon content of 37 %; and a mix of 2/1 wt% ratio [16].

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2.2 Synthesis procedures

2.2.1 General method for the glycidylation (functionalization with epoxy groups) of bio-oil

The glycidylation of bio-oil was conducted following the method and conditions established by

[17,18]. The described process leads to the synthesis of epoxyalkylaryl ethers by the reaction

between compounds containing acid hydroxyl groups, and haloepoxyalkanes in the presence of a

strong alkali. A reactor was loaded with the desired amount of bio-oil (containing a certain

amount of moles of -OH / gram of oil) and epichlorohydrin (5 moles epichlorohydrin / mole -

OH). The mixture was heated at 100°C and benzyltriethylammonium chloride (0.05 moles

BnEt3NCl / mole -OH) was added. After 1 h, the resulting suspension was cooled down to 30°C,

and an aqueous solution of NaOH 20 wt% (2 moles NaOH / mole –OH and also containing 0.05

moles of BnEt3NCl / mole –OH) was added dropwise. Once the addition of the alkaline aqueous

solution was completed, the mixture was vigorously stirred for 90 min at 30°C, and then the

reaction was stopped. As soon as the reaction was completed, the crude product was filtered to

remove the insoluble matter. After that, the mixture was poured into an extraction funnel, the

organic layer was separated; and through numerous liquid-liquid extractions with deionized

water, the organic phase was abundantly washed until neutral pH was reached. Finally, the

organic phase was vacuum concentrated and the reaction yield was gravimetrically determined.

A similar procedure was carried out to functionalize the α-resorcylic acid (or α-RA) with epoxy

groups. Refer to Chapter III for more details about this synthesis.

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2.3. Characterization of Starting and Modified Materials

2.3.1 Gas Chromatography - Mass Spectrometry (GC-MS)

Chemical composition of bio-oil was analyzed with an Agilent 7890 GC/5975MS using a DB-

1701 column (30 m; 0.25 mm i.d.; 0.25 mm film thickness). A representative bio-oil sample

(~150 mg) was weighed and mixed with 3 mL of methanol and diluted to 10 mL with

dichloromethane. The dilute sample was injected into the column and each sample was injected

twice. The initial temperature of the column, 40°C, was maintained for 2 min, and the

temperature was subsequently increased to 250°C at 5°C/ min, and the final temperature was held

for 8 min. Helium of ultra-high purity (99.99%) supplied from Airgas Inc. (Charlotte, NC) was

used as a carrier gas and flowed at 1.25 mL/min. The injector and the GC/MS interface were

kept at constant temperature of 280 and 250°C, respectively. Compounds were ionized at 70 eV

electron impact conditions and analyzed over a mass per change (m/z) range of 50–550. Bio-oil

compounds were identified by comparing the mass spectra with the NIST (National Institute of

Standards and Technology) mass spectral library [19].

2.3.2 Hydroxyl group analysis through 31

P-NMR spectroscopy.

Since the aim of this work is to functionalize the aromatic structures of the lignin fragments and

other phenolic compounds present in the bio-oil with reactive epoxy groups (by reaction with

epichlorohydrin), it is of great importance to quantify the content of phenolic hydroxyl groups

(mmol of phenolic -OH / gram of bio-oil). This determination allows establishing the amount of

epichlorohydrin needed for the reaction. The type of -OH groups present in the bio-oil was

determined and quantitatively calculated using 31P-NMR. For lignin and lignin derived products,

the most common used phosphytylation reagent is 2-chloro-4,4,5,5-tetramethyl-1,3,2-

178

dioxaphospholane (TMDP), because it provides better signal resolutions when collecting the

spectra; as well as higher stability of the phophitylated products. This compound reacts with

hydroxyl groups arising from aliphatic, phenolic, and carboxylic groups in the presence of an

organic base (such as pyridine) to give phosphylated products, whose presence can be quantified

through 31P-NMR spectroscopy [20]. Prior to 31P-NMR analysis, bio-oils were phosphitylated

with 150 mL of TMDP and then the spectra were acquired with a Bruker Avance II 250 MHz

according to the method reported by David et al. [21]. In order to assess the extent of the

functionalization of the aromatic structures of the lignin fragments and other phenolic

compounds present in the bio-oil with reactive epoxy groups, 31P-NMR was performed again to

the reaction product to verify the removal of phenolic hydroxyl groups after chemical reaction

with epichlorohydrin in alkaline conditions.

2.3.3 Fourier Transform Infrared Spectroscopy (FTIR)

With the aim to study the functional groups present in the product, FTIR was performed. Infrared

spectra of the starting materials and glycidylated products were measured by attenuated total

reflection (ATR) method using a Thermo Nicolet 6700 Fourier transform infrared spectrometer

connected to a PC with OMNIC software analysis. All spectra were recorded between 400 and

4000 cm-1 over 64 scans with a resolutions of 16 cm-1.

2.3.4 Epoxide equivalent weight determination.

Epoxide equivalent weight (EEW, g/eq.) of the epoxy products was determined by epoxy

titration according to Pan et al. [22]. Briefly, a solution of 0.1 M HBr in glacial acetic acid was

179

used as a titrant, 1 wt% crystal violet in glacial acetic acid was used as indicator, and toluene as

solvent for the sample. The EEW was calculated using Equation:

EEW = 0111 6 �7 6 8 (5.1)

where W is the sample weight (g), N is the molarity of HBr solution, and V is the volume of HBr

solution used for titration (mL).

2.4 General procedure for the formulation of crosslinked epoxy networks.

In order to generate a cross-linked polymeric network, epoxy monomers can undergo a chain

homopolymerization in the presence of both Lewis acids and bases such as tertiary amines

(process known as anionic homopolymerization). Glycidylated bio-oil (or epox. bio-oil) and

triglycidylated ether of α-resorcylic acid (or epox. α-RA), used as a comonomer, were mixed in

different weight proportions. After that, 4-dimethylaminopyridine or DMPA (a Lewis base) was

added to the mixtures in a ratio of 0.08 mole DMAP / mole epoxy [23]. The mixtures were then

poured in an aluminum mold and cured at 120°C for 12 hrs and post cured for 2 hrs at 160°C.

2.5 Characterization of the epoxy networks

2.5.1 Soxhlet extraction.

Soxhlet extraction was employed to characterize the structures of the bulk copolymers following

the ASTM D5369-93 [24]. An appropriate amount of the bulk polymer was extracted for 24 h

with 200 mL of refluxing dichloromethane with a Soxhlet extractor. Upon extraction, the

resulting solution was concentrated by rotary evaporation and subsequent vacuum drying. The

soluble substances were isolated for further characterization. The insoluble solid was dried in a

vacuum oven for several hours before it was weighed.

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2.5.2 Scanning Electron Microscopy (SEM)

SEM study was performed with the purpose of obtaining a topological characterization of the

materials. The fractured surfaces of pre-chilled samples in liquid nitrogen were studied by

scanning electron microscope (Zeiss EVO 50 variable pressure scanning electron microscope

with digital imaging and EDS). Samples were sputtered with gold prior to SEM observations

with an EMS 550X auto sputter coating device with carbon coating attachment.

2.5.3 Thermo-mechanical properties of the networks.

Dynamic mechanical analysis (DMA) on a TA Instruments RSA III was carried out to assess the

thermo-mechanical properties by three-point bending. The tests were performed at temperatures

ranging from 25 to 250°C with a heating rate of 10°C/min. The frequency was fixed a 1Hz and a

sinusoidal strain-amplitude of 0.1 % was used for the analysis. The dynamic storage modulus

(E′) and tan δ curves were plotted as a function of temperature. To be able to explain the effects

of the co-monomers on the mechanical properties, it is useful to calculate the crosslinking

density (n, mol/m³). The crosslinking density can be estimated from the experimental data using

the rubber elasticity theory. Thermosets behave as rubbers above Tg. At small deformations,

rubber elasticity predicts that the modulus storage (E’), of an ideal elastomer with a network

structure is proportional to the crosslinking density according to Equation 5.2 [25]:

�� = 3��� = 3��/�� (5.2)

where E` is the rubbery storage modulus, R is the gas constant (8.314 J/mol K), T is the absolute

temperature (K), ρ is the density of the sample (g/m³), and Mc is the molecular weight between

crosslinks (g/mol). The temperature and rubbery modulus were determined for the calculation of

181

the equation at Tg + 30°C. The temperature at which the peak of the tan delta presents a

maximum was considered as the glass transition temperature of the material. The Fox equation

was employed to fit the experimental Tg data reported in the results. It serves as a way to

describe Tg of the multicomponent networks as a function of relative epoxy monomer content.

The Fox expression is shown in Equation 5.3 [26]:

0

[\ = �0[\0 + �F

[\F (5.3)

In this expression, W1 and W2 represent weight fractions of components 1 and 2 (bio-oil and α-

resorcylic acid, respectively), and Tg1 and Tg2 are Tg values of polymer samples containing only

components 1 and 2.

3. Results and Discussion

3.1 Gas Chromatography - Mass Spectrometry (GC-MS)

According to Laurichesse et al. [8], during the lignification process, the monolignol units are

linked together via radical coupling reactions to form a complex three-dimensional molecular

architecture, containing a great variety of bonds, with typically 50% of β-O-4 ether linkages.

During pyrolysis, biomass is heated at around 500°C without oxygen. Lignin starts to decompose

between 280-500°C by the cleavage of ether and carbon-carbon linkages. The components have

been classified into the main categories of hydroxyaldehydes, hydroxyketones, sugars,

carboxylic acids and phenolics. The concentration of phenol is typically very low (~ 0.1%),

while GC analyzable monomeric phenols commonly range around 1- 4%. Most of the phenolics

are too high in molecular weight to be quantified by GS analysis, and they are present as

oligomers containing varying numbers of acidic, phenolic and carboxylic acid hydroxyl groups,

as well as aldehyde, alcohol, and ether functions. The molecular weight of these oligomers

182

ranges from hundreds to 5000 g/mol [2]. However, in order to study the composition of the fast

pyrolysis bio-oil, a GC/MS analysis was performed.

Figure 5.2 shows the resulting total ion chromatogram. Thirty nine compounds including

phenolic monomers, fatty acids, sugars, ketones and organic acids were detected by GC/MS from

fast pyrolysis bio-oil. The products are identified in Table 5.2, along with their retention time

(R.T.) and their respective peak area %.

Figure 5.2. Total ion GC/MS chromatogram of fast pyrolysis bio-oil.

1

23 4

5 6 7

89

1011

12

1413

15

1617

181920

21

2223

24

2526

2728

29

30

3132

33

34

353637

38

39

0.E+00

1.E+05

2.E+05

3.E+05

4.E+05

5.E+05

6.E+05

7.E+05

8.E+05

9.E+05

1.E+06

0 5 10 15 20 25 30 35 40 45 50

Ab

un

da

nce

Retention Time (minutes)

Phenolic Compounds

183

Table 5.2. Major pyrolysis decomposition products identified by GS/MS

Peak Number Compound R.T. (min) Peak Area (%)

1 Acetic acid 4.402 3.36

2 Methoxycyclobutane 8.036 0.59

3 2-cyclopenten-1-one 8.494 0.58

4 Furfural 8.540 2.24

5 2-methyl-2-cyclopenten-1-one 10.159 0.87

6 4,5-dihydro-2-methyl-1H-imidazole 13.432 0.79

7 2,3- dimethyl-2-cyclopenten-1-one 14.662 0.68

8 3-methyl-1,2-cyclopentanedione 14.891 2.09

9 Phenol 15.996 1.56

10 2-methoxyphenol 16.156 2.65

11 2-methylphenol 17.289 1.65

12 2-methoxy-3-methylphenol 18.124 0.60

13 3-methylphenol 18.307 0.84

14 4-methylphenol 18.365 1.01

15 2-methoxy-4-methylphenol 19.000 2.32

16 2,4-dimethylphenol 19.520 1.09

17 5-methoxy-2,3-dimethylphenol 20.716 0.99

18 4-ethyl-2-methoxyphenol 21.231 1.15

19 m-methoxybenzamide 22.759 1.07

20 4-allyl-2-methoxyphenol 23.325 1.33

21 2,6-dimethoxyphenol 24.115 6.58

22 2-methoxy-4-[(1 Z)-1-propenyl]-phenol 24.727 1.56

23 2-methoxy-4-[(1 E)-1-propenyl)]-phenol 25.980 1.94

24 Dehydroacetic acid 26.335 3.96

25 4-Hydroxy-3-methoxybenzaldehyde 26.535 1.94

26 1,2,3-trimethoxy-5-methylbenzene 28.063 2.02

27 1-(3-hydroxy-4-methoxyphenyl)-ethanone 28.429 1.45

28 2,5-dimethoxy-4-ethylamphetamine 29.396 1.17

29 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid 29.797 3.21

30 2-hydroxyimino-N-(4-methoxyphenyl)-acetamide 30.936 1.66

31 D-allose 31.800 5.20

32 2,6-dimethoxy-4-(2-propenyl-1)-phenol 32.206 2.88

33 4-hydroxy-3,5-dimethoxybenzaldehyde 32.847 5.71

34 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone 34.254 2.49

35 4-Hydroxy-3-methoxycinnamaldehyde 34.975 1.24

36 1-(2,4,6-trihydroxy-3-methylphenyl)-1-butanone 35.113 1.39

37 2-methyl-9H-carbazole 35.759 0.70

38 Hexadecanoic acid 36.343 10.67

39 6-octadecenoic acid 39.582 16.78

184

Phenolic compounds are mainly derived from the thermal degradation of lignin. Based on the

results of the GC/MS analysis, the phenolic compounds present on the bio-oil can be classified

into three fractions (Table 5.3), comprising guaiacol derivatives, syringol derivatives, and finally

other phenolics.

Table 5.3. Fractions of phenolic compounds found in bio-oil

Guaiacol derivatives Syringol derivatives Other phenolics

2-methoxyphenol 2,6-dimethoxy-phenol 3-methylphenol

2-methoxy-3-methylphenol 1-(3-hydroxy-4-methoxyphenyl)-ethanone

4-methylphenol

2-methoxy-4-methylphenol 2,6-dimethoxy-4-(2-propenyl-1)-phenol

2,4-dimethylphenol

2-methoxy-4-methylphenol 4-hydroxy-3,5-dimethoxybenzaldehyde

1,2,3-trimethoxy-5-methylbenzene

4-allyl-2-methoxyphenol 1-(4-hydroxy-3,5-dimethoxyphenyl)-ethanone

Phenol

2-methoxy-4-[(1 Z)-1-propenyl]-phenol

2-methylphenol

2-methoxy-4-[(1 E)-1-propenyl)]-phenol

5-methoxy-2,3-dimethylphenol

4-Hydroxy-3-methoxybenzaldehyde

3-(4-hydroxy-3-methoxyphenyl)-2-propenoic

acid

4-Hydroxy-3-methoxycinnamaldehyde

According to the results, phenolic compounds account for about 48 % of all compounds detected

and identified by the GS/MS analysis, being the guaiacol derivatives fraction the most numerous

of them. The syringyl/guaiacyl ratio (S/G) was calculated by dividing the sum of the peak areas

of syringyl units by the sum of the peak areas of guaiacyl units [27]. A ratio value of around S/G

= 1.14 was obtained. It can be inferred from this value that there is a majority of S units than G

units. As stated by Nonier et al. [27], up to 500°C, S units of lignins are more sensitive to the

185

pyrolysis reaction compared to G units, which require higher temperatures for thermal

decomposition. This behavior leads to mixtures of phenolic derivatives richer in syringyl

compounds. As previously stated, most of the phenolics present in the crude bio-oil are too high

in molecular weight to be quantified by GS analysis. They are present as oligomers containing

varying numbers of different organic functional groups.

3.2 Hydroxyl group analysis through 31

P-NMR spectroscopy.

31P-NMR method involves the phosphitylation of hydroxyl groups in a substrate using a 31P

reagent followed by quantitative 31P-NMR analysis. The type of -OH groups present in the bio-

oil was determined and quantitatively calculated using 31P-NMR. The results are shown in Table

5.4. According to the 31P-NMR analysis, the total hydroxyl number for freshly produced fast

pyrolysis bio-oil was (10.83 ± 1.08) mmol/g. It was found that aliphatic, phenolic and acidic

hydroxyl groups accounted for 49 %, 28 %, and 23 % of the total –OH number, respectively.

Aliphatic –OH are likely to arise as a result of the degradation of cellulose, hemicellulose and

lignin; whist the presence of phenolic –OH can be attributed to the cleavage of ether bonds in

linin during thermal decomposition.

186

Table 5.4. Hydroxyl number (OHN) of pyrolysis oil determined by quantitative 31P-NMR

Hydroxyl Number (OHN) of raw and modified bio-oil.

Raw Bio-oil

(fresh)

Raw Bio-oil

(stored 5

months)

Modified Bio-

oil

Aliphatic -OH

5.33 1.31 3.10

Ph

eno

lic

OH

C5 substituted Condensed phenolic -OH

β-5 0.04 0.35 0.00

4-O-5 0.18 0.72 0.00

5-5 1.28 0.48 0.00

Guaiacyl phenolic -OH

0.44 0.22 0.00

Catechol type -OH

0.76 0.74 0.00

p-hydroxy-phenyl -OH

0.33 0.51 0.00

Acidic -OH

2.48 0.75 0.00

Total -OHN ( ±1.08

mmol/g) 10.83 5.08 3.10

Aging of bio-oil is a commonly observed process in pyrolysis bio-oil. With the aim to determine

whether or not there was any change in the hydroxyl number, the analysis was repeated after

storing the bio-oil for 5 months at 4 °C. It can clearly be observed that after this period, the total

–OHN decreased to about the half of that shown by freshly produced bio-oil. As stated by Yang,

Kumar and Huhnke [28], bio-oils are an intermediate product, in which (due to thermodynamic

instability) some species are still highly active, leading to changes in their chemical and physical

properties. In the particular case of alcohols and acids, it seems to be likely that formation of

esters by esterification of alcohols and acids (or by transesterification of two esters), addition

reactions of alcohols to aldehydes or ketones to form hemiacetals or acetals, oxidation of

alcohols, etc. might be playing a substantial role in lowering the total –OHN of the bio-oil after

storing. It is very important to mention that, for this period of time, the total content of phenolic

187

–OH remained invariant (around 3.03 mmol/g); suggesting that these compounds seemed to play

a secondary role in bio-oil aging.

3.3 General method for the glycidylation (functionalization with epoxy groups) of bio-oil.

3.3.1 Glycidylation of the α-resorcylic acid

In order to study the reaction between polyphenols and the epichlorohydrin, α-resorcylic acid

(containing two phenolic hydroxyl groups and one carboxylic acid moiety), was employed as a

representative model. For details about this reaction refer to Chapter III.

3.3.2 Glycidylation of the bio-oil.

Phenols can be converted into glycidyl ethers by reaction with 2,3-epoxyalkyl halides in alkaline

solution. The condensation of phenolic compounds with epichlorohydrin, catalyzed by bases, has

been the subject of study since long time ago [29]. In alkaline solutions, phenols exist as the

phenoxide ion. As stated by Auvergne et al. [30], the reaction between the strongly nucleophilic

phenoxide ions and epichlorohydrin can follow two competitive mechanisms: a one-step

nucleophilic substitution, in which the phenoxide ion attacks the halide and displaces a halide ion

(Clˉ in this case) through a SN2 mechanism (widely known as the Williamson synthesis of

ethers); and a two-step mechanism involving a nucleophilic attack of the epoxide (due to the ease

of opening of the strained three-membered ring) to form 1-aryloxy-3-chloropropan-2-ols. The

second step comprises the subsequent regeneration of the epoxide ring through intramolecular

cyclization of the previously formed 1-aryloxy-3-chloropropan-2-ols. From Tomita`s patent [31],

which covers and details the procedure to obtain epoxy resins derived from gallic acid and

tannins (by introducing glycidyl groups to carboxyl and phenols groups through reaction with

188

epichlorohydrin in alkaline media), it can be inferred that, under the disclosed experimental

conditions, the two-step reaction mechanism seems to be favored. In the first step, the addition of

epichlorohydrin to carboxyl and hydroxyl groups, at high temperature, takes place; and during

the second step of the process, the dehydrogenation of the halohydrin ether and ester moieties

obtained in the first step is achieved by adding an alkaline aqueous solution. It is also important

to highlight that the presence of a phase transfer catalyst is essential for the addition reaction in

the first step.

Since the main goal of this research lies in developing bio-based thermosetting materials, a

similar approach was used to functionalize the phenolic compounds found in the bio-oil. Once

the total –OHN was determined, the glycidylation reaction was performed. The following

scheme (Figure 5.3) depicts the reaction of a hypothetical lignin fragment with epichlorohydrin

in alkaline medium.

Figure 5.3. Glycidylation of bio-oil with epichlorohydrin in alkaline medium.

As previously detailed, most of the chemistry of phenols is similar to that of alcohols. Thus,

phenols can be converted into ethers by reaction with alkyl halides in the presence of a base.

Since the formation of the reactive phenoxide ion occurs more readily than the alkoxide ion, the

OH

O H

O H

O

CH3

O CH3

O

L

OH

OH

O

O

CH3

O CH3

O

L

OO

C l

1) 100 °C, 1h, BnEt

3NCl

2) NaOH (aq),

BnEt NCL = Lignin Fragments

189

acid phenolic –OH groups are the preferred target of the reaction. As seen in Table III, 31P-NMR

analysis of the modified pyrolysis oil revealed that the phenolic –OH groups were totally

removed. Titration with HBr showed that this compound has an average molar epoxy content of

(3.0 ± 0.1) x 10-3 mol per gram of resin, corresponding to an epoxy equivalent weight = 333 g/eq.

This value is approx. the half of the content shown by commercial epoxy resins, such as the well-

known EPON 828, having an average molar epoxy content of approx. 5.3 x 10-3 mol per gram of

resin, resulting in an epoxy equivalent weight = 185-192 g/eq. During purification and

recovering of the modified resin, around 40 wt% is recovered as epoxidized bio-oil. The content

of insoluble matter, mainly char, is around 30 wt% and the remaining 30 wt% is lost during

purification (water soluble compounds such as acids, low molecular weight alcohols and

carbonyl compounds, dehydrated sugars and other products of cellulose pyrolysis). This is also

reflected in the results of the 31P-NMR determination, where the highly hydrophilic acidic –OH

compounds (mainly acetic acid) are completely lost after modification and subsequent washing

and purification. Aliphatic –OH content increase is likely due to mass concentration, after

elimination of acid compounds and insoluble matter.

3.4 Fourier Transform Infrared Spectroscopy (FTIR)

With the aim to study the functional groups present in the product, FTIR was performed. The

results are shown in Figure 5.4. The FTIR spectrum of bio-oil was characterized by the presence

of a strong IR absorption band at 3370 cm-1 (due to the presence of hydroxyl groups from

alcohols, acids, and phenols); as well as an intense carbonyl strong peak at 1750 cm-1 (mainly

attributed to ketones, aldehydes and acids). In the case of the modified bio-oil, there is a decrease

in the intensity of the absorption band at 3370 cm-1, related to the removal of phenolic and acid -

190

OH functional groups (due to the glycidylation reaction), as well as the elimination of water

during purification. In the spectrum of the modified oil, it is also evident the presence of

absorption bands at 1227.5 cm-1 and 1050 cm-1, attributed to the presence of -C-O-C- groups

formed during the reaction, as well as the characteristic bands for oxirane rings at 847.5 cm-1 and

910 cm-1, confirming the presence of epoxy groups in the modified mixture.

Figure 4. FTIR spectra of raw bio-oil, modified bio-oil and epichlorohydrin.

Figure 5.4. FTIR spectra of raw bio-oil, modified bio-oil and epichlorohydrin.

4. Characterization of the epoxy networks

4.1 Soxhlet Extraction

Soxhlet extraction was employed to determine the total solvent extractable content of the bulk

copolymers. An appropriate amount of the bulk polymer was extracted for 8 h with 250 mL of

refluxing acetone with a Soxhlet extractor. Upon extraction, the insoluble solid was dried in a

O

Cl

Epichlorohydrin

-Cl O

Bio-Oil modified with epoxy groups

Raw Bio-Oil

O

OH

OH

O

O

CH3

O CH3

O

L

O

-C-O-C-

-OH

-C-H -OH

191

vacuum oven for several hours before it was weighted. The obtained results are displayed in

Figure 5.5.

Figure 5.5. Mass loss (wt%) of bio-oil / α-resorcylic acid epoxy resins

As seen in the plot, the extracted fraction ranged from about 20 wt% to 29 wt% when exposing

the materials to organic extraction. An increase in the amount of extractable content was

observed when increasing the amount of epoxidized bio-oil in the mixtures. This behavior could

be related to two different factors. On the one hand, it is clear that after post-curing, an average

value of 1/4 of the monomers remained unreacted (in terms of weight). Once the system reaches

gelation and the post gel stage, the local mobility of the functional groups is considerably

shortened, severely impacting the kinetics of the reaction; and therefore decreasing the monomer

conversion [32]. For the low molecular weight (and highly reactive epox. α-resorcylic acid

monomers), around 20 wt% of the total content is not included in the epoxy network. This

phenomenon can have even a deeper impact in the conversion of the functionalized phenolic

compounds found in the bio-oil, which have a more limited mobility than their epox. α-resorcylic

100 %

Epox. RA

70 % Epox. RA /

30 % Epox. Bio-Oil

50 % Epox. RA /

50 % Epox. Bio-Oil

25 % Epox. RA /

75 % Epox. Bio-Oil

100 %

Epox. Bio-Oil

0

5

10

15

20

25

30

Epoxy Systems

Ex

tra

cta

ble

co

nte

nt

(%)

192

acid counterpart (due to their larger size and higher molecular weight), leading to a lesser

incorporation of monomers into the growing network. On the other hand, as previously stated,

bio-oil consists is a complex mixture of a large number of pyrolysis compounds. After chemical

modification and subsequent purification, many non-functionalized compounds remain in the

polymerizable mixture. After curing, there is a population of compounds, not able to react,

showing chemical affinity for the acetone; and therefore, extractable, resulting in a higher

amount of soluble products when the proportion of bio-oil was increased in the mixtures.

4.2 Scanning Electron Microscopy (SEM)

Scanning electron microscopy was used to investigate the morphology of the epoxy systems.

Figure 5.6 provides the obtained images of the fracture surfaces of the bio-oil / α-RA materials,

pre-chilled in liquid nitrogen. As shown, the fracture surfaces of the materials containing

epoxidized α-resorcylic acid are characterized by smooth, glassy and homogeneous

microstructures; without any plastic deformation [33,34]. They exhibited surfaces with cracks

and different planes, indicating brittle fracture. As the proportion of this low molecular weight

co-monomer decreases, the brittle character of the materials also decreased, which is reflected in

the more ductile surface of the 100 wt% epoxidized bio-oil sample, which exhibits some degree

of plastic deformation. Likewise, globular cavities dispersed in the continuous epoxy matrix are

observed, probably due to evaporation of residual solvent used during the polymerization stage.

193

Figure 5.6. SEM micrographs of the 100 wt% epoxidized bio-oil (5.6.a), 50 wt% epoxidized bio-

oil – 50 wt% epoxidized α-RA (5.6.b), and 100 wt% epoxidized α-RA (5.6.c) resins.

5.6.a

5.6.b

5.6.c

194

4.3 Thermo-mechanical characterization of bio-oil-based epoxy thermosetting resin

With the aim of minimizing the use of commercial agents as a part of the epoxy resin

formulation, the polymeric materials were prepared by anionic polymerization initiated by

DMPA, in a ratio of 0.08 mol DMAP/mol epoxy. In order to prepare the resins, epoxidized bio-

oil and triglycidylated ether of α-resorcylic acid (as a low molecular weight reactive comonomer)

were mixed in different weight proportions. After curing, the polymeric materials appeared as

dark-brown stiff materials at room temperature. Dynamic mechanical analysis (DMA)

measurements are intensively used to investigate the amorphous phase transitions of polymers as

they are stressed under periodic deformation. Results are shown in Figure 5.7.

100 % Epox. RA

90% Epox. RA - 10%

Epox. Bio-Oil

70% Epox RA - 30%

Epox. Bio-oil

50% Epox. RA - 50%

Epox. Bio-oil

25% Epox. RA- 75%

Epox.Bio-Oil100% Epox. Bio-Oil

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

25 50 75 100 125 150 175 200 225 250

Sto

rag

e M

od

ulu

s (

Pa

)

Temperature (Celsius)

7-a

195

Figure 5.7. Temperature dependence of the storage modulus (5.7.a) and loss factor or tan δ

(5.7.b) as a function of the composition of the epoxy resins

Results revealed that the materials behaved as thermosets. As seen in the curves, the storage

modulus initially remained approximately constant. As the temperature increased, the storage

modulus exhibited a drop over a wide temperature range (glass transition), followed by a

modulus plateau at high temperatures, where the material behaved like a rubber. This region,

called the rubbery plateau, evidences the presence of a stable cross-linked network. The modulus

drop is associated with the beginning of segmental mobility in the cross-linked polymer network.

As can be seen, the transition between the glassy to the rubbery state was wide for the tested

materials. This transition is highly influenced by the bio-oil content in the formulations, and it

ranged from around 100°C for pure epox. bio-oil, up to approx. 180°C for the 100 wt% α-RA

(with the mixes of these two components falling in between the above mentioned values).

Concerning the glass transition, the peak in tan δ was used as a criterion to determine its value.

100% Epox. RA

90% Epox. RA - 10%

Epox. Bio-Oil

70% Epox. RA - 30%

Epox. Bio-Oil

50% Epox. RA - 50%

Epox. Bio-Oil

25% Epox. RA - 75%

Epox. Bio-Oil

100% Epox.Bio-Oil

0.00

0.10

0.20

0.30

0.40

0.50

0.60

50 75 100 125 150 175 200 225 250

Ta

n δ

Temperature (Celsius)

7-b

196

The results showed that the peaks shifted to lower temperatures (indicating a decrease in the

glass transition temperature), when the proportion of epoxidized bio-oil was increased. The peak

width at half height is a criterion used to indicate the interaction between the phases and

homogeneity of the amorphous phase. A higher value implies higher inhomogeneity of the

amorphous phase. In this particular case, the results showed that the tan δ peak became wider as

the bio-oil content in the matrix increased. This phenomenon could be associated to the presence

of less cross-linked and heterogeneous networks with wider molecular weight distribution of the

chains between crosslinking points [35]. Likewise, all the materials showed only one tan δ peak,

indicating that there was no phase separation. Table 5.5 summarizes the main properties of the

bio-oil-based epoxy thermosets.

Table 5.5. Thermo-mechanical properties of epoxidized bio-oil and the triglycidylated ether of α-

resorcylic acid (epoxidized α-resorcylic) resins

Epoxidized

bio-oil (wt%)

Triglycidylated ether

of α-resorcylic acid

(wt%)

E` at 30°C

(Pa)

Tg

(°C)

Density of active chains

(mol/m3)

100 0 1.48E+09 131.2 1006

75 25 1.58E+09 138.5 9169

50 50 2.18E+09 176.7 13702

30 70 2.32E+09 183.6 15238

10 90 2.74E+09 206.3 27150

0 100 3.78E+09 212.2 35709

197

As shown in Table 5.5, the storage modulus (E’) exhibited values in the order of GPa, indicating

that materials behave as glassy polymers at ambient conditions; and it considerably decreased

when the epoxidized bio-oil wt% content was increased. This parameter for glassy polymers

below the glass transition temperature is approx. constant, having a value of around 3 x 109 Pa

[36]. The decline in the modulus values is directly correlated to the decrease in the density of

active chains or n; parameter that is often associated to the crosslinking density of the material.

As the crosslinking density increases, the hardness, modulus, mechanical strength, and chemical

resistance increase [37]. For instance, slightly crosslinked materials, such as typical rubber

bands, have n = 190 mol/m3 [36]. When the same rubber is highly crosslinked, it generates a

brittle material. A common commercial diglycidyl ether of bisphenol A (DGEBA) type epoxy

resin (epoxy equivalent weight = 185-192 g/eq. [38]) used in coating, adhesive, structural, and

composite applications shows an approximate value of n = 5000 mol/m3 when crosslinked with a

tetrafunctional amine [39]. Conceptually, the higher the crosslinking density, the more

constrained the chain motion; and thus the stiffer and more thermally resistant the molecular

structure is. As seen in Table 5.5, the value of n increased approximately 37 times, when

comparing the pure epoxidized materials (100 wt% bio-oil and 100 wt% α-resorcylic acid).

When more α-RA is incorporated into the bio-oil, the n value increased, leading to more

crosslinked materials; which improved their thermo-mechanical properties, as evidenced by the

rise of the Tg and modulus. Such behavior could be related to the fact that the average epoxy

content of the epoxidized derivatives of the α-resorcylic acid is 3 times larger than the one shown

by the epox. bio-oil, leading to a higher capability to crosslink. Likewise, structural differences

between the epoxidized bio-oil monomers, as well as differences in the molecular weights, might

play a key role in the way they are incorporated into the network during polymerization.

198

The glass transition temperature was also highly influenced by the epox. bio-oil wt% content,

ranging from approx. 130 to 212 °C, showing its maximum value for the 100 α-resorcylic acid

wt%. The Fox equation was used to fit the experimental data. Figure 5.8 is a plot of 1/Tg versus

relative epoxy bio-oil content. This equation assumes uniform morphology; therefore, the good

agreement (R2 = 0.9654) with the bio-oil / α-resorcylic acid epoxy systems implies that the

networks are relatively homogeneous due to good miscibility between the components [40].

Figure 5.8. Relationship between 1/Tg and relative epoxy bio-oil weight content (Wbio-oil)

In order to compare the results obtained in this research, the obtained thermo-mechanical

properties of the 100 wt% epoxidized bio-oil and 100 wt% epoxidized α-RA were contrasted to

those shown by a commercial epoxy resin (Super Sap 100 Epoxy by Entropy Resins)

polymerized by both condensation with a commercial amine and by anionic polymerization with

DMPA. Table 5.6 summarizes the main properties these epoxy thermosets.

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1 /

Tg

(K-1

)

Epox. Bio-Oil Weight Fraction (Wbio-oil)

Fox Equation

Experimental Data

199

Table 5.6. Mechanical properties of the epoxidized bio-oil, triglycidylated α-RA and Super Sap

100 epoxy

Resin

E` at 30°C

(Pa)

Tg

(°C)

Density of active

chains (mol/m3)

100 wt% Ttriglycidylated α-RA 2.90E+09 212.2 35709

100 wt% Epox. Bio-oil 1.48E+09 131.2 9169

Super Sap 100 epoxy resin

homopolymerized with DMPA 1.53E+09 75.4 731

Super Sap 100/1000 epoxy resin

and hardener 5.33E+08 58.9 641

Results revealed that the properties of the mechanical properties of epoxidized bio-oil resin and

the triglycidylated α-RA based resin are considerably better than those shown by their

commercial counterpart, exhibiting modulus values in the order of ~ 1 – 2 GPa at room

temperature and glass transition temperatures above 100°C, typical of a thermoset used for

composite applications.

200

4. Conclusions

Bio-oil pyrolysis phenolic derivatives, as well as α-resorcylic acid, were chemically

functionalized with epoxy monomers through glycidylation with epichlorohydrin in alkaline

conditions. The phenolic compounds were identified by GS/MS, and the total hydroxyl number

(after and before chemical modification) was successfully calculated through 31P-NMR. The

epoxy content of the bio-oil upon chemical functionalization was measured by means of a

titration and found to be (3.0 ± 0.1) x 10-3 mol per gram of resin. Grafting of epoxy functions

onto the monomer`s structure was confirmed by FTIR. Testing showed an increase in the storage

modulus, from approx. 1.5 to 3.8 GPa at room temperature, when higher amounts of

triglycidylated ether of α-resorcylic acid were added to the mixtures. Glass transition

temperature appeared to be strongly dependent of the amounts of triglycidylated ether of α-

resorcylic acid when it was combined with the epoxidized bio-oil. When compared to samples of

commercial origin, testing showed the thermo-mechanical properties of the epoxidized bio-oil /

triglycidylated α-RA based resins are considerably higher to those shown by their commercial

counterpart. Based on the results of this study, it is clear that these materials behave as glassy

polymers at room conditions; and they could be used as thermosetting matrices for the

development of composite materials. Likewise, they have potential as partial replacements in the

formulation of commercial epoxy resins used as coatings, adhesives, etc. This study opens the

possibility of using these kinds of pyrolysis bio-oils as renewable precursors for polymer

synthesis.

201

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General conclusions

• Thermosetting polymers where obtained from biomass derived materials such as

vegetable oils, and terpenes, as well as from biorefinery products. The obtained resins

could be employed as partial replacements of thermosetting commercial resins, and/or in

the formulation of composite materials.

• From the experimental results, it was clear that the chemical modification of different

functional groups of biomass derived materials considerably improved the thermo-

mechanical performance of the obtained polymers, when compared to their non-modified

counterparts.

• Studies of the polymerization reactions revealed that the homopolymerization of epoxy

resins using anionic chemical initiators stands as a viable option to generate materials

with higher contributions of biomass-derived compounds per weight basis

• The vast thermo-mechanical analysis of the resulting thermosets revealed the deep

existing link between the thermo-mechanical response of the materials and their chemical

structure. Stiffer materials were obtained when the density of aromatic units were

increased in the crosslinked networks. Likewise, properties were also dependent on the

chemical nature and position of the polymerizable functional groups.

• A significant chemical analysis, both quantitative and qualitative, has been devoted to the

study of pyrolysis bio-oil of lignocellulosic biomass as a source of chemicals for the

synthesis of thermosetting resins.

207

• Overall results indicated that the materials presented thermo-mechanical behaviors

similar to those shown by commercial formulations.

Conclusions highlighting the major achievements of Chapter II:

• Thermo-mechanical analysis revealed that the cationic copolymerization of tung oil and

terpenes in presence of boron trifluoride yielded elastomeric materials at room

temperature. For all the studied comonomers, suitable materials for analysis were not

obtained for terpene proportions above 60 wt%.

• The obtained copolymers presented a maximum in their thermo-mechanical properties,

showing a modulus of approx. 38 MPa at room temperature and a glass transition

temperature of around 30 °C. The Tg was found to be a function of both the type and

comonomer weight content.

• Studies revealed that the properties of non-chemically modified vegetable oils can change

as a function of two different effects of the fatty acid chains, either by plasticizing the

network because of their flexible structure, or by increasing the rigidity due to the large

number of unsaturations per molecule.

• Some of the thermo-mechanical properties were found to be predictable by simple

mathematical approaches and models developed for polyvinyl resins.

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Conclusions highlighting the major achievements of Chapter III:

• Thermo-mechanical tests indicated that the chemical incorporation of polymerizable

groups onto the structure of the vegetable oils considerably improved their mechanical

performance. The maximum glass transition obtained in Chapter II was doubled, and the

modulus increased by a fold of approximately 37 times.

• The inclusion of α-resorcylic acid as a low molecular weight comonomer increased the

chain stiffness and therefore improved the thermo-mechanical behavior, rendering resins

with properties matching those shown by glassy thermosetting commercial resins.

• When polymerizing the epoxidized derivatives of linseed oil and α-resorcylic acid,

thermodynamical incompatibility was found for oil proportions higher than 30 wt%. A

new method to improve the mixture of these two materials is exposed in Chapter IV.

• Studies also revealed that the chain-growth copolymerization allowed increasing the

contribution of renewable materials into the formulations, by minimizing the use of

commercial crosslinking compounds.

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Conclusions highlighting the major achievements of Chapter IV:

• Results revealed that interpenetrating of the system containing an epoxy phase derived

from α-resorcylic acid, and an acrylate phase derived from vegetable oil arises as an

approach to not only improve the properties by including an aromatic monomer in the

system; but also to minimize the phase separation phenomena by kinetically controlling

this thermodynamical process by permanent interlocking of entangled chains, avoiding

phase separation.

• Thermo-mechanical characterization showed that the obtained materials behave as

thermoset amorphous polymers, and that both the modulus and glass transition are

extremely dependent on the epoxy/acrylate weight ratio, as well as on the chemical

structure of the epoxy monomers.

• Modulus values ranged from 0.7 to 3.3 GPa at 30 °C, and glass transition temperatures

ranged from around 58 °C to approx. 130 °C. No synergistic effect on these two

properties was observed.

• Interpenetrating networks containing equivalent weight proportions of the parent resins

showed the highest fracture toughness of the series, exhibiting a KIc value of around 2.1

MPa·m1/2. The results showed that the KIc values did not increase as Mc increased, which

seems to suggest that a different mechanism is responsible for the increase in the fracture

toughness displayed by IPNs. Also, there seems to be an exponential type increase in the

fracture energy with the Mc1/2

for the materials containing the epoxy phase

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Conclusions highlighting the major achievements of Chapter V:

• Biomass refinery stands as a sustainable way for aromatics biosourcing, by providing low

molecular weight fragments of natural lignin, whose functional groups are more exposed

to the reaction medium (due to their lesser steric crowding), and thus being readily

reachable during chemical synthesis.

• According to the results, phenolic compounds account for about 48 % of all compounds

detected and identified by the GS/MS analysis, being the guaiacol derivatives fraction the

most numerous of them.

• According to the 31P-NMR analysis, the total hydroxyl number for freshly produced fast

pyrolysis bio-oil was (10.83 ± 1.08) mmol/g. It was found that aliphatic, phenolic and

acidic hydroxyl groups accounted for 49 %, 28 %, and 23 % of the total –OH number,

respectively.

• Testing showed an increase in the storage modulus, from approx. 1.5 to 3.8 GPa at room

temperature, when higher amounts of triglycidylated ether of α-resorcylic acid were

added to the mixtures. Glass transition temperature appeared to be strongly dependent of

the amounts of triglycidylated ether of α-resorcylic acid when it was combined with the

epoxidized bio-oil. 100 wt% epoxidized bio-oil was characterized by a glass transition of

approx. 130 °C.

211

Future Work

A considerable part of this document deals with the formulation of thermosetting materials,

especially with epoxy resins. Nature provides many bio-based feedstocks such as vegetable oils

and polyphenols to obtain bio-based epoxy resins; and research and studies have focused on the

formulation of epoxidized products, but no crosslinking or curing agents. It is essential that all

components of the formulation are both renewable and sustainable. Therefore, it is of great

importance to develop curing agents from renewable resources rather than petroleum-derived

materials for producing the next generation of bio-based epoxy formulations. There is a

considerable potential to perform research on this area, which can catapult bio-base epoxy resin

to the commercial markets, especially on amine hardeners, which production remains as a

challenge.

As explained in the general introduction, composite materials will be a sector highly influenced

by the evolution in the polymer and materials industry. Therefore, more research needs to be

performed concerning the application of new bio-based derived materials in the design and

formulation of composite materials.

Bio-based chemicals are already industrially available but most of them are aliphatic or

cycloaliphatic regarding their chemical structure. However, as stated in this document, many key

chemical compounds are aromatic, which in our current times are ultimately derived from

petroleum. Thus, there is a challenge for finding aromatic building blocks derived from biomass.

212

In nature, aromatics are mostly found in lignins; however some crucial aspects need to be

improved if these products want to be taken to the commercialization stage, such as:

• Due to the inherited variability of the chemical composition of lignocellulosic materials,

as well as pretreatment techniques, pyrolysis products and their composition are highly

variable, making hard the comparison of results and approaches to convert them into

added-value products. The field needs standard protocols for the extraction procedures,

pretreatments, catalysts, analysis techniques and procedures, so as to make results and

approaches possibly comparable.

• Further fundamental understanding of both the structure and the chemistry of the

products of fast pyrolysis of biomass is required for assessment of their reactivity under

different reaction conditions. This will allow extracting meaningful and valuable

information for the structure-property-reactivity relations of the obtained materials.

• In the case of pyrolysis techniques, more experimental work is required to determine

optimal operation conditions, such as catalysts, catalysts deactivations, temperature,

pressure to control product yield. Moreover, purification and separation of the chemical

compounds present in pyrolysis bio-oils remains as a major challenge. Adequate isolation

of certain products or fractions can vastly improve the analysis and study of their

structure and reactivity.

213

• Catalysis stands up as a key technology for biomass conversion, especially for the

valorization of lignin and its derivatives. Development of more efficient catalytic

technologies, increasing the selectivity of the conversion toward the obtention of

particular products or family of products remains as a challenge.

• Particularly for aromatic bio-sourcing, pyrolysis techniques should be applied directly to

lignins. In this way, more selective and richer fractions in aromatic can be obtained,

eliminating the influence and presence of decomposition products of the hemicellulose

and cellulose.